Assessment of regional long-axis function during dobutamine echocardiography

Echocardiographic analysis of regional left ventricular function is based upon the assessment of radial motion. Long-axis motion is an important contributor to overall function. but has been difficult to evaluate clinically until the recent development of tissue Doppler techniques. We sought to compare the standard visual assessment of radial motion with quantitative tissue Doppler measurement of peak systolic velocity. timing and strain rate (SRI) in 104 patients with known or suspected coronary artery disease undergoing dobutamine stress echocardiography (DbE). A standard DbE protocol was used with colour tissue Doppler images acquired in digital cine-loop format. peak systolic velocity (PSV), time to peak velocity (TPV) and SRI were assessed off-line by an independent operator. Wall motion was assessed by an experienced reader. Mean PSV, TPV and SRI values were compared with wall motion and the presence of coronary artery disease by angiography. A further analysis included assessing the extent of jeopardized myocardium by comparing average values of PSV, TPV and SRI against the previously validated angiographic score. Segments identified as having normal and abnormal radial wall motion showed significant differences in mean PSV (7.9 +/- 3.8 and 5.9 +/- 3.3 cm/s respectively; P < 0.001), TPV (84 40 and 95 +/- 48 ms respectively; P = 0.005) and SRI (- 1.45 +/- 0.5 and - 1.1 +/- 0.9 s(-1) respectively; P < 0.001). The presence of a stenosed subtending coronary artery was also associated with significant differences from normally perfused segments for mean PSV (8.1 3.4 compared with 5.7 +/- 3.7 cm/s; P < 0.001), TPV (78 50 compared with 92 +/- 45 ms; P < 0.001) and SRI (- 1.35 0.5 compared with - 1.20 +/- 0.4 s(-1); P = 0.05). PSV, TPV and SRI also varied significantly according to the extent of jeopardized myocardium within a vascular territory. These results suggest that peak systolic velocity, timing of contraction and SRI reflect the underlying physiological characteristics of the regional myocardium during DbE, and may potentially allow objective analysis of wall motion.

Differential inhibition of human CYP1A1 AND CYP1A2 by quinidine and quinine.

The inhibition of recombinant CYP1A1 and CYP1A2 activity by quinidine and quinine was evluated using ethoxyresorufin O -deethylation, phenacetin O -deethylation and propranolol desisopropylation as probe catalytic pathways. 2. With substrate concentrations near the K m of catalysis, both quinidine and quinine potently inhibited CYP1A1 activity with [ I ] 0.5 ~ 1-3 μM, whereas in contrast, there was little inhibition of CYP1A2 activity. The Lineweaver-Burk plots with varying inhibitor concentrations suggested that inhibition by quinidine and quinine was competitive. 3. There was only trace metabolism of quinidine by recombinant CYP1A1, whereas rat liver microsomes as a control showed extensive consumption of quinidine and metabolite production. 4. This work suggests that quinidine is a non-classical inhibitor of CYP1A1 and that it is not as highly specific at inhibiting CYP2D6 as previously thought.

Dietary treatment of iron deficiency in women of childbearing age

Background: The Australian Iron Status Advisory Panel advocates dietary intervention as the first treatment option for mild iron deficiency [serum ferritin (SF) = 10-15 mug/L]. However, there appear to be no studies on the efficacy of dietary treatment for iron deficiency. Objective: We compared the effects of iron supplementation and of a high-iron diet on serum ferritin (SF) and hemoglobin in iron-deficient women of childbearing age. Design: Forty-four iron-deficient women (SF < 15 mug/L or SF = 15-20 mug/L plus serum iron < 10 mu mol/L and total-iron-binding capacity > 68 mu mol/L) and 22 iron-replete women (hemoglobin greater than or equal to 120 g/L and SF > 20 mug/L) matched for age and parity categories were enrolled and completed 7-d weighed food records at baseline. The iron-deficient women were randomly allocated to receive iron supplementation (105 mg/d; supplement group) or a high-iron diet (recommended intake of absorbable iron: 2.25 mg/d; diet group) for 12 wk. Hematologic and dietary assessments were repeated at the end of the intervention and again after a 6-mo follow-up. Results: Mean SF in the supplement group increased from 9.0 +/- 3.9 mug/L at baseline to 24.8 +/- 10.0 mug/L after the intervention and remained stable during follow-up (24.2 +/- 9.8 mug/L whereas the diet group had smaller increases during the intervention (8.9 +/- 3.1 to 11.0 +/- 5.9 mug/L) but continued to improve during follow-up (to 15.2 +/- 9.5 mug/L). Mean hemoglobin tended to improve in both intervention groups, but the change was only significant in the supplement group. Conclusions: In iron-deficient women of childbearing age, a high-iron diet produced smaller increases in SF than did iron supplementation but resulted in continued improvements in iron status during a 6-mo follow-up.

Solution structures by H-1 NMR of the novel cyclic trypsin inhibitor SFTI-1 from sunflower seeds and an acyclic permutant

SFTI-1 is a recently discovered cyclic peptide trypsin inhibitor from sunflower seeds comprising 14 amino acid residues. It is the most potent known Bowman-Birk inhibitor and the only naturally occurring cyclic one. The solution structure of SFTI-1 has been determined by H-1-NMR spectroscopy and compared with a synthetic acyclic permutant. The solution structures of both are remarkably similar. The lowest energy structures from each family of 20 structures of cyclic and acyclic SFTI-1 have an rmsd over the backbone and heavy atoms of 0.29 Angstrom and 0.66 Angstrom, respectively. The structures consist of two short antiparallel beta -strands joined by an extended loop containing the active site at one end. Cyclic SFTI-1 also has a hairpin turn completing the cycle. Both molecules contain particularly stable arrangements of cross-linking hydrogen bonds between the beta -strands and a single disulfide bridge, making them rigid and well defined in solution. These stable arrangements allow both the cyclic and acyclic variants of SFTI-1 to inhibit trypsin with very high potencies (0.5 nM and 12.1 nM, respectively). The cyclic nature of SFTI-1 appears to have evolved to provide higher trypsin inhibition as well as higher stability. The solution structures are similar to the crystal structure of the cyclic inhibitor in complex with trypsin. The lack of a major conformational change upon binding suggests that the structure of SFTI-1 is rigid and already pre-organized for maximal binding due to minimization of entropic losses compared to a more flexible ligand. These properties make SFTI-1 an ideal platform for the design of small peptidic pharmaceuticals or pesticides. (C) 2001 Academic Press.

Adaptation to chronic eccentric exercise in humans: the influence of contraction velocity

We compared changes in muscle fibre composition and muscle strength indices following a 10 week isokinetic resistance training programme consisting of fast (3.14 rad(.)s(-1)) or slow (0.52 rad(.)s(-1)) velocity eccentric muscle contractions. A group of 20 non-resistance trained subjects were assigned to a FAST (n = 7), SLOW (n = 6) or non-training CONTROL (n = 7) group. A unilateral training protocol targeted the elbow flexor muscle group and consisted of 24 maximal eccentric isokinetic contractions (four sets of six repetitions) performed three times a week for 10 weeks. Muscle biopsy samples were obtained from the belly of the biceps brachii. Isometric torque and concentric and eccentric torque at 0.52 and 3.14 rad(.)s(-1) were examined at 0, 5 and 10 weeks. After 10 weeks, the FAST group demonstrated significant [mean (SEM)] increases in eccentric [29.6 (6.4)%] and concentric torque [27.4 (7.3) %] at 3.14 rad(.)s(-1), isometric torque [21.3 (4.3)%] and eccentric torque [25.2 (7.2) %] at 0.52 rad(.)s(-1). The percentage of type I fibres in the FAST group decreased from [53.8 (6.6)% to 39.1 (4.4)%] while type lib fibre percentage increased from [5.8 (1.9)% to 12.9 (3.3)%; P < 0.05]. In contrast. the SLOW group did not experience significant changes in muscle fibre type or muscle torque. We conclude that neuromuscular adaptations to eccentric training stimuli may be influenced by differences in the ability to cope with chronic exposure to relatively fast and slow eccentric contraction velocities. Possible mechanisms include greater cumulative damage to contractile tissues or stress induced by slow eccentric muscle contractions.

The nature and evolution of the association among digeneans, molluscs and fishes

Patterns of association of digenean families and their mollusc and vertebrate hosts are assessed by way of a new database containing information on over 1000 species of digeneans for lift-cycles and over 5000 species from fishes. Analysis of the distribution of digenean families in molluscs suggests that the group was associated primitively with gastropods and that infection of polychaetes, bivalves and scaphopods are all the results of host-switching. For the vertebrates. infections of agnathans and chondrichthyans are apparently the result of host-switching from teleosts. For digenean families the ratio of orders of fishes infected to superfamilies of molluscs infected ranges from 0.5 (Mesometridae) to 16 (Bivesiculidae) and has a mean of 5.6. Individual patterns of host association of 13 dipenean families and superfamilies are reviewed. Two, Bucephalidae and Sanguinicolidae. are exceptional in infecting a range of first intermediate hosts qualitatively as broad as their range of definitive hosts. No well-studied taxon shows narrower association with vertebrate than with mollusc clades. The range of definitive hosts of digeneans is characteristically defined by eco-physiological similarity rather than phylogenetic relationship. The range of associations of digenean families with mollusc taxa is generally much narrower. These data are considered in the light of ideas about the significance of different forms of host association. If Manter's Second Rule (the longer the association with a host group, the mure pronounced the specificity exhibited by the parasite group) is invoked, then the data may suggest that the Digenea first parasitised molluscs before adopting vertebrate hosts. This interpretation is consistent with most previous ideas about the evolution of the Digenea but contrary to current interpretations based on the monophyly of the Neodermata. The basis of Manter's Second Rule is. however, considered too flimsy for this interpretation to be robust. Problems of the inference of the evolution of patterns of parasitism in the Neodermata al-e discussed and considered so intractable that the truth may be presently unknowable. (C) 2001 Australian Society for Parasitology Inc. Published by Elsevier Science Ltd. All rights reserved.

Effects of high-dose etidronate treatment on microdamage accumulation and biomechanical properties in beagle bone before occurrence of spontaneous fractures

We recently demonstrated that suppressed bone remodeling allows microdamage to accumulate and causes reductions in some mechanical properties. However, in our previous study, I year treatment with high-dose etidronate (EHDP) did not increase microdamage accumulation in most skeletal sites of dogs in spite of complete remodeling suppression and the occurrence of spontaneous fractures of ribs and/or thoracic spinous processes. This study evaluates the effects of EHDP on microdamage accumulation and biomechanical properties before fractures occur. Thirty-six female beagles, 1-2 years old, were treated daily for 7 months with subcutaneous injections of saline vehicle (CNT) or EHDP at 0.5 (E-low) or 5 mg/kg per day (E-high). After killing, bone mineral measurement, histomorphometry, microdamage analysis, and biomechanical testing were performed. EHDP treatment suppressed intracortical and trabecular remodeling by 60%-75% at the lower dose, and by 100% at the higher dose. Osteoid accumulation caused by a mineralization deficit occurred only in the E-high group, and this led to a reduction of mineralized bone mass. Microdamage accumulation increased significantly by two- to fivefold in the rib, lumbar vertebra, ilium, and thoracic spinous process in E-low, and by twofold in the lumbar vertebra and ilium in E-high. However, no significant increase in damage accumulation was observed in ribs or thoracic spinous processes in E-high where fractures occur following 12 months of treatment. Mechanical properties of lumbar vertebrae and thoracic spinous processes were reduced significantly in both E-low and E-high. These findings suggest that suppression of bone remodeling by EHDP allows microdamage accumulation, but that osteoid accumulation reduces production of microdamage. (Bone 29:271-278; 2001) (C) 2001 by Elsevier Science Inc. All rights reserved.

Effects of suppressed bone turnover by bisphosphonates on microdamage accumulation and biomechanical properties in clinically relevant skeletal sites in beagles

We recently demonstrated that suppression of bone remodeling allows microdamage to accumulate, leading to reduced bone toughness in the rib cortex of dogs. This study evaluates the effects of reduced bone turnover produced by bisphosphonates on microdamage accumulation and biomechanical properties at clinically relevant skeletal sites in the same dogs. Thirty-six female beagles, 1-2 years old, were divided into three groups. The control group was treated daily for 12 months with saline vehicle (CNT), The remaining two groups were treated daily with risedronate at a dose of 0.5 mg/kg per day (RIS), or alendronate at 1.0 mg/kg per day (ALN) orally, The doses of these bisphosphonates were six times the clinical doses approved for treatment of osteoporosis in humans. After killing, the L-1 vertebra was scanned by dual-energy X-ray absorptiometry (DXA), and the L-2 vertebra and right ilium were assigned to histomorphometry, The L-3 vertebra, left ilium, Th-2 spinous process, and right femoral neck were used for microdamage analysis. The L-4 vertebra and Th-1 spinous process were mechanically tested to failure in compression and shear, respectively. One year treatment with risedronate or alendronate significantly suppressed trabecular remodeling in vertebrae (RIS 90%, ALN 95%) and ilium (RIS 76%, ALN 90%) without impairment of mineralization, and significantly increased microdamage accumulation in all skeletal sites measured. Trabecular bone volume and vertebral strength increased significantly following 12 month treatment. However, normalized toughness of the L-4 vertebra was reduced by 21% in both RIS (p = 0.06) and ALN (p = 0.05) groups. When the two bisphosphonate groups were pooled in a post hoc fashion for analysis, this reduction in toughness reached statistical significance (p = 0.02), This study demonstrates that suppression of trabecular bone turnover by high doses of bisphosphonates is associated with increased vertebral strength, even though there is significant microdamage accumulation and a reduction in the intrinsic energy absorption capacity of trabecular bone. (C) 2001 by Elsevier Science Inc. All rights reserved.

Seedling responses of three Australian tree species to toxic concentrations of zinc in solution culture

A frequently desired outcome when rehabilitating Zn toxic sites in Australia is to establish a self-sustaining native ecosystem. Hence, it is important to understand the tolerance of Australian native plants to high concentrations of Zn. Very little is known about the responses of Australian native plants, and trees in particular, to toxic concentrations of Zn. Acacia holosericea, Eucalyptus camaldulensis and Melaleuca leucadendra plants were grown in dilute solution culture for 10 weeks. The seedlings (42 days old) were exposed to six Zn treatments viz., 0.5, 5, 10, 25, 50 and 100 muM. The order of tolerance to toxic concentrations of Zn was E. camaldulensis > A. holosericea > M. leucadendra, the critical external concentrations being approximately 20, 12 and 1.5 muM, respectively. Tissue Zn concentrations increased as solution Zn increased for all species. Root tissue concentrations were higher than shoot tissue concentrations at all solution Zn concentrations. The critical tissue Zn concentrations were approximately 85 and 110 mug g(-1) DM for M. leucadendra, 115 and 155 mug g(-1) DM for A. holosericea and 415 and 370 mug g(-1) DM for E. camaldulensis for the youngest fully expanded leaf and total shoots, respectively. The results from this paper provide the first comprehensive combination of growth responses, critical external concentrations, critical tissue concentrations and plant toxicity symptoms for three important Australian genera, viz., Eucalyptus, Acacia and Melaleuca, for use in the rehabilitation of potentially Zn toxic sites.

Randomised controlled trial of telemedicine for new neurological outpatient referrals

Objective-To test the hypothesis that telemedicine for new patient referrals to neurological outpatients is as efficient and acceptable as conventional face to face consultation. Methods-A randomised controlled trial between two groups: face to face (FF) and telemedicine (TM). This study was carried out between a neurological centre and outlying clinics at two distant hospitals linked by identical medium cost commercial interactive video conferencing equipment with ISDN lines transmitting information at 384 kbits/s. The same two neurologists carried out both arms of the study. Of the 168 patients who were suitable for the study, 86 were randomised into the telemedicine group and 82 into the face to face group. Outcome measures were (I) consultation process: (a) number of investigations; (b) number of drugs prescribed; (c) number of patient reviews and (2) patient satisfaction: (a) confidence in consultation; (b) technical aspects of consultation; (c) aspects surrounding confidentiality. Diagnostic categories were also measured to check equivalence between the groups: these were structural neurological, structural non-neurological, nonstructural, and uncertain. Results-Diagnostic categories were similar (p>0.5) between the two groups. Patients in the telemedicine group had significantly more investigations (p=0.001). There was no difference in the number of drugs prescribed (p>0.5). Patients were generally satisfied with both types of consultation process except for concerns about confidentiality and embarrassment in the telemedicine group (p=0.017 and p=0.005 respectively). Conclusion-Telemedicine for new neurological outpatients is possible and feasible but generates more investigations and is less well accepted than face to face examination.

Adrenocorticotropin stimulation tests in patients with hypothalamic-pituitary disease: low dose, standard high dose and 8-h infusion tests

Objectives Low doses of ACTH [1-24] (0.1, 0.5 and 1.0 mug per 1.73 m(2)) may provide a more physiological level of adrenal stimulation than the standard 250 mug test, but not all studies have concluded that the 1.0 fig is a more sensitive screening test for central hypoadrenalism. Eight-hour infusions of high dose ACTH [1-24] have also been suggested as a means of assessing the adrenals' capacity for sustained cortisol secretion. In this study, we compared the diagnostic accuracy of three low dose ACTH tests (LDTs) and the 8-h infusion with the standard 250 jig test (HDT) and the insulin hypoglycaemia test (IHT) in patients with hypothalamic-pituitary disease. Subjects and design Three groups of subjects were studied. A healthy control group (group 1, n=9) and 33 patients with known hypothalamic or pituitary disease who were divided into group 2 (n=12, underwent IHT) and group 3 (n=21, IHT contraindicated). Six different tests were performed: a standard IHT (0.15 U/kg soluble insulin); a 60-minute 250 mug HDT; three different LDTs using 0.1 mug, 0.5 mug and 1.0 mug (all per 1.73 m(2)); and an 8-h infusion test (250 mug ACTH [1-24] at a constant rate over 8 h). Results Nine out of the 12 patients in group 2 failed the IHT. Three out of 12 patients from group 2 who clearly passed the IHT, also passed all the ACTH [1-24] stimulation tests. Seven of the 9 patients who failed the lHT, failed by a clear margin (peak cortisol

Liveweight gains of steers at different stocking rates on monospecific Gatton panic and Estrella grass pastures in the Chaco Central region of Paraguay

A grazing trial to study the effect of stocking rate on animal production and botanical composition of Gallon panic (Panicum maximum) and Estrella grass (Cynodon nlemfuensis) was conducted in the central region of the Paraguayan Chaco between 1992 and 1998. The experiment included 6 stocking rates (0.5, 0.8. 1.1, 1.4. 1.7 and 2.0 AU/ha) on individual 4-ha paddocks. The pasture treatments were continuously grazed by yearling steers. replaced annually, over a 4-year grazing period. No fertiliser was used. Botanical composition was recorded annually in autumn from 1992 to 1998 while animal production data were recorded monthly from 1992 to 1996. Relationships between animal productivity and stocking rates were determined by regression analysis. Gallon panic produced greater liveweight gains per head than Estrella grass at low and intermediate stocking rates. However, the slope of the linear relationship between liveweight gain per head and stocking rate increased each year in Gallon panic indicating that the productivity of this grass progressively declined at higher stocking rates over the period of observation. Estrella grass showed less sensitivity to stocking rate but was affected severely by periods of low rainfall.

ENCORE: The effect of nutrient enrichment on coral reefs. Synthesis of results and conclusions

Coral reef degradation resulting from nutrient enrichment of coastal waters is of increasing global concern. Although effects of nutrients on coral reef organisms have been demonstrated in the laboratory, there is little direct evidence of nutrient effects on coral reef biota in situ. The ENCORE experiment investigated responses of coral reef organisms and processes to controlled additions of dissolved inorganic nitrogen (N) and/or phosphorus (P) on an offshore reef(One Tree Island) at the southern end of the Great Barrier Reef, Australia. A multi-disciplinary team assessed a variety of factors focusing on nutrient dynamics and biotic responses. A controlled and replicated experiment was conducted over two years using twelve small patch reefs ponded at low tide by a coral rim. Treatments included three control reefs (no nutrient addition) and three + N reefs (NH4Cl added), three + P reefs (KH2PO4 added), and three + N + P reefs. Nutrients were added as pulses at each low tide (ca twice per day) by remotely operated units. There were two phases of nutrient additions. During the initial, low-loading phase of the experiment nutrient pulses (mean dose = 11.5 muM NH4+; 2.3 muM PO4-3) rapidly declined, reaching near-background levels (mean = 0.9 muM NH4+; 0.5 muM PO4-3) within 2-3 h. A variety of biotic processes, assessed over a year during this initial nutrient loading phase, were not significantly affected, with the exception of coral reproduction, which was affected in all nutrient treatments. In Acropora longicyathus and A. aspera, fewer successfully developed embryos were formed, and in A. longicyathus fertilization rates and lipid levels decreased. In the second, high-loading, phase of ENCORE an increased nutrient dosage (mean dose = 36.2 muM NH4+; 5.1 muM PO4-3 declining to means of 11.3 muM NH4+ and 2.4 muM PO4-3 at the end of low tide) was used for a further year, and a variety of significant biotic responses occurred. Encrusting algae incorporated virtually none of the added nutrients. Organisms containing endosymbiotic zooxanthellae (corals and giant clams) assimilated dissolved nutrients rapidly and were responsive to added nutrients. Coral mortality, not detected during the initial low-loading phase, became evident with increased nutrient dosage, particularly in Pocillopora damicornis. Nitrogen additions stunted coral growth, and phosphorus additions had a variable effect. Coral calcification rate and linear extension increased in the presence of added phosphorus but skeletal density was reduced, making corals more susceptible to breakage. Settlement of all coral larvae was reduced in nitrogen treatments, yet settlement of larvae from brooded species was enhanced in phosphorus treatments. Recruitment of stomatopods, benthic crustaceans living in coral rubble, was reduced in nitrogen and nitrogen plus phosphorus treatments. Grazing rates and reproductive effort of various fish species were not affected by the nutrient treatments. Microbial nitrogen transformations in sediments,were responsive to nutrient loading with nitrogen fixation significantly increased in phosphorus treatments and denitrification increased in all treatments to which nitrogen had been added. Rates of bioerosion and grazing showed no significant effects of added nutrients, ENCORE has shown that reef organisms and processes investigated ill situ were impacted by elevated nutrients. Impacts mere dependent on dose level, whether nitrogen and/or phosphorus mere elevated and were often species-specific. The impacts were generally sub-lethal and subtle and the treated reefs at the end of the experiment mere visually similar to control reefs. Rapid nutrient uptake indicates that nutrient concentrations alone are not adequate to assess nutrient condition of reefs. Sensitive and quantifiable biological indicators need to be developed for coral reef ecosystems. The potential bioindicators identified in ENCORE should be tested in future research on coral reef/nutrient interactions. Synergistic and cumulative effects of elevated nutrients and other environmental parameters, comparative studies of intact vs. disturbed reefs, offshore vs, inshore reefs, or the ability of a nutrient-stressed reef to respond to natural disturbances require elucidation. An expanded understanding of coral reef responses to anthropogenic impacts is necessary, particularly regarding the subtle, sub-lethal effects detected in the ENCORE studies. (C) 2001 Published by Elsevier Science Ltd.

Influence of fungal pathogens and environmental conditions on disease severity, flower fall and desiccation of harvested Geraldton waxflower - 2. Studies with commercial packages

Relationships were examined between environmental conditions mediated by packaging and handling and the deterioration of harvested Geraldton waxflower cv. 'Fortune Cookie'. Disease severity plus flower and leaf drop caused by inoculation with Botrytis cinerea were reduced by lowering handling temperatures to 0, 5 or 5/20 degreesC alternated daily, versus 20 degreesC. They were also reduced by inhibition of ethylene action with a silver thiosulfate pulse pretreatment. Additionally, treatments that enhanced water loss, such as packing dry, keeping forced air-cooling holes open and strategic placement of extra ventilation holes may also reduce disease severity and flower plus leaf fall. Inclusion of KMnO4-based Bloomfresh ethylene scrubbing sachets in packages did not reduce disease severity or lessen flower plus leaf fall. Thus, deterioration of waxflower packaged in commercial cartons can be minimised by keeping temperatures low, packing plant material dry, use of cartons with strategically placed ventilation holes and/or pretreatment with silver thiosulfate.

Effect of chronic clonidine treatment on transmitter release from sympathetic varicosities of the guinea-pig vas deferens

1 Previous studies have demonstrated that chronic pre-synaptic inhibition of transmitter release by morphine evokes a counter-adaptive response in the sympathetic nerve terminals that manifests itself as an increase in transmitter release during acute withdrawal. In the present study we examined the possibility that other pre-synaptically acting drugs such as clonidine also evoke a counter-adaptive response in the sympathetic nerve terminals. 2 In chronically saline treated (CST) preparations, clonidine (0.5 muM) completely abolished evoked transmitter release from sympathetic varicosities bathed in an extracellular calcium concentration ([Ca2+](o)) of 2 mM. The inhibitory effect of clonidine was reduced by increasing [Ca2+](o) from 2 to 4 mM and the stimulation frequency from 0.1 to 1 Hz. 3 The nerve terminal impulse (NTI) was not affected by concentrations of clonidine that completely abolished evoked transmitter release. 4 Sympathetic varicosities developed a tolerance to clonidine (0.5 muM) following 7-9 days of chronic exposure to clonidine. 5 Acute withdrawal of preparations following chronic clonidine treatment (CCT) resulted in a significant (P