Lead reduces tension development and the myosin ATPase activity of the rat right ventricular myocardium

Lead (Pb2+) poisoning causes hypertension, but little is known regarding its acute effects on cardiac contractility. To evaluate these effects, force was measured in right ventricular strips that were contracting isometrically in 45 male Wistar rats (250-300 g) before and after the addition of increasing concentrations of lead acetate (3, 7, 10, 30, 70, 100, and 300 ��M) to the bath. Changes in rate of stimulation (0.1-1.5 Hz), relative potentiation after pauses of 15, 30, and 60 s, effect of Ca2+ concentration (0.62, 1.25, and 2.5 mM), and the effect of isoproterenol (20 ng/mL) were determined before and after the addition of 100 ��M Pb2+. Effects on contractile proteins were evaluated after caffeine treatment using tetanic stimulation (10 Hz) and measuring the activity of the myosin ATPase. Pb2+ produced concentration-dependent force reduction, significant at concentrations greater than 30 ��M. The force developed in response to increasing rates of stimulation became smaller at 0.5 and 0.8 Hz. Relative potentiation increased after 100 ��M Pb2+ treatment. Extracellular Ca2+ increment and isoproterenol administration increased force development but after 100 ��M Pb2+ treatment the force was significantly reduced suggesting an effect of the metal on the sarcolemmal Ca2+ influx. Concentration of 100 ��M Pb2+ also reduced the peak and plateau force of tetanic contractions and reduced the activity of the myosin ATPase. Results showed that acute Pb2+ administration, although not affecting the sarcoplasmic reticulum activity, produces a concentration-dependent negative inotropic effect and reduces myosin ATPase activity. Results suggest that acute lead administration reduced myocardial contractility by reducing sarcolemmal calcium influx and the myosin ATPase activity. These results also suggest that lead exposure is hazardous and has toxicological consequences affecting cardiac muscle.

Eucalyptol, an essential oil, reduces contractile activity in rat cardiac muscle

Eucalyptol is an essential oil that relaxes bronchial and vascular smooth muscle although its direct actions on isolated myocardium have not been reported. We investigated a putative negative inotropic effect of the oil on left ventricular papillary muscles from male Wistar rats weighing 250 to 300 g, as well as its effects on isometric force, rate of force development, time parameters, post-rest potentiation, positive inotropic interventions produced by Ca2+ and isoproterenol, and on tetanic tension. The effects of 0.3 mM eucalyptol on myosin ATPase activity were also investigated. Eucalyptol (0.003 to 0.3 mM) reduced isometric tension, the rate of force development and time parameters. The oil reduced the force developed by steady-state contractions (50% at 0.3 mM) but did not alter sarcoplasmic reticulum function or post-rest contractions and produced a progressive increase in relative potentiation. Increased extracellular Ca2+ concentration (0.62 to 5 mM) and isoproterenol (20 nM) administration counteracted the negative inotropic effects of the oil. The activity of the contractile machinery evaluated by tetanic force development was reduced by 30 to 50% but myosin ATPase activity was not affected by eucalyptol (0.3 mM), supporting the idea of a reduction of sarcolemmal Ca2+ influx. The present results suggest that eucalyptol depresses force development, probably acting as a calcium channel blocker.

Ventricular performance and Na+-K+ ATPase activity are reduced early and late after myocardial infarction in rats

Myocardial infarction leads to compensatory ventricular remodeling. Disturbances in myocardial contractility depend on the active transport of Ca2+ and Na+, which are regulated by Na+-K+ ATPase. Inappropriate regulation of Na+-K+ ATPase activity leads to excessive loss of K+ and gain of Na+ by the cell. We determined the participation of Na+-K+ ATPase in ventricular performance early and late after myocardial infarction. Wistar rats (8-10 per group) underwent left coronary artery ligation (infarcted, Inf) or sham-operation (Sham). Ventricular performance was measured at 3 and 30 days after surgery using the Langendorff technique. Left ventricular systolic pressure was obtained under different ventricular diastolic pressures and increased extracellular Ca2+ concentrations (Ca2+e) and after low and high ouabain concentrations. The baseline coronary perfusion pressure increased 3 days after myocardial infarction and normalized by 30 days (Sham 3 = 88 �� 6; Inf 3 = 130 �� 9; Inf 30 = 92 �� 7 mmHg; P < 0.05). The inotropic response to Ca2+e and ouabain was reduced at 3 and 30 days after myocardial infarction (Ca2+ = 1.25 mM; Sham 3 = 70 �� 3; Inf 3 = 45 �� 2; Inf 30 = 29 �� 3 mmHg; P < 0.05), while the Frank-Starling mechanism was preserved. At 3 and 30 days after myocardial infarction, ventricular Na+-K+ ATPase activity and contractility were reduced. This Na+-K+ ATPase hypoactivity may modify the Na+, K+ and Ca2+ transport across the sarcolemma resulting in ventricular dysfunction.

Reactivity of the isolated perfused rat tail vascular bed

Isolated segments of the perfused rat tail artery display a high basal tone when compared to other isolated arteries such as the mesenteric and are suitable for the assay of vasopressor agents. However, the perfusion of this artery in the entire tail has not yet been used for functional studies. The main purpose of the present study was to identify some aspects of the vascular reactivity of the rat tail vascular bed and validate this method to measure vascular reactivity. The tail severed from the body was perfused with Krebs solution containing different Ca2+ concentrations at different flow rates. Rats were anesthetized with sodium pentobarbital (65 mg/kg) and heparinized (500 U). The tail artery was dissected near the tail insertion, cannulated and perfused with Krebs solution plus 30 ��M EDTA at 36oC and 2.5 ml/min and the procedures were started after equilibration of the perfusion pressure. In the first group a dose-response curve to phenylephrine (PE) (0.5, 1, 2 and 5 ��g, bolus injection) was obtained at different flow rates (1.5, 2.5 and 3.5 ml/min). The mean perfusion pressure increased with flow as well as PE vasopressor responses. In a second group the flow was changed (1.5, 2, 2.5, 3 and 3.5 ml/min) at different Ca2+ concentrations (0.62, 1.25, 2.5 and 3.75 mM) in the Krebs solution. Increasing Ca2+ concentrations did not alter the flow-pressure relationship. In the third group a similar protocol was performed but the rat tail vascular bed was perfused with Krebs solution containing PE (0.1 ��g/ml). There was an enhancement of the effect of PE with increasing external Ca2+ and flow. PE vasopressor responses increased after endothelial damage with air and CHAPS, suggesting an endothelial modulation of the tone of the rat tail vascular bed. These experiments validate the perfusion of the rat tail vascular bed as a method to investigate vascular reactivity.

Effects of ouabain on vascular reactivity

Ouabain is an endogenous substance occurring in the plasma in the nanomolar range, that has been proposed to increase vascular resistance and induce hypertension. This substance acts on the a-subunit of Na+,K+-ATPase inhibiting the Na+-pump activity. In the vascular smooth muscle this effect leads to intracellular Na+ accumulation that reduces the activity of the Na+/Ca2+ exchanger and to an increased vascular tone. It was also suggested that circulating ouabain, even in the nanomolar range, sensitizes the vascular smooth muscle to vasopressor substances. We tested the latter hypothesis by studying the effects of ouabain in the micromolar and nanomolar range on phenylephrine (PE)-evoked pressor responses. The experiments were performed in normotensive and hypertensive rats in vivo, under anesthesia, and in perfused rat tail vascular beds. The results showed that ouabain pretreatment increased the vasopressor responses to PE in vitro and in vivo. This sensitization after ouabain treatment was also observed in hypertensive animals which presented an enhanced vasopressor response to PE in comparison to normotensive animals. It is suggested that ouabain at nanomolar concentrations can sensitize vascular smooth muscle to vasopressor stimuli possibly contributing to increased tone in hypertension.

Indicadores de qualidade do solo e determina����o de n��veis de degrada����o de pastagens

A utiliza����o do solo no Brasil foi realizada de forma explorat��ria, com a convers��o de sistemas naturais em sistemas agr��colas extrativistas. Grande parte das ��reas de sistemas naturais deu lugar ��s ��reas de cultivo, posteriormente sucedidas por pastagens, encontrando-se boa parte em elevado est��gio de degrada����o. Diante do exposto, o presente trabalho tem como objetivo avaliar a sensibilidade de alguns atributos f��sicos, qu��micos, compartimentos da mat��ria org��nica e determina����es de campo como indicadores de qualidade do solo estabelecendo rela����es entre os mesmos. O estudo foi desenvolvido no munic��pio de Governador Valadares-MG, para tal foram escolhidos n��veis de pastagens progressivamente degradadas observadas visualmente (pastagem 1, pastagem 2, pastagem 3 e pastagem 4), duas ��reas de capoeira em est��gios de regenera����o natural (capoeira 1 e capoeira 2) e mata (refer��ncia). O solo em estudo foi um Argissolo Vermelho, textura argilosa. As determina����es dos indicadores f��sicos, qu��micos e compartimentos da mat��ria org��nica foram realizadas em quatro profundidades (0-5, 5-10, 10-20 e 20-40 cm). Foram realizadas tamb��m determina����es de campo, todos os atributos foram determinados no ter��o m��dio de uma pedoforma convexa, em dois per��odos, chuvoso e seco. Atrav��s dos atributos do solo utilizados como indicadores do solo, foi poss��vel separar dois n��veis de pastagens degradadas, baixa degrada����o (pastagem 1 e pastagem 2) e elevada degrada����o (pastagem 3 e pastagem 4). A melhor qualidade do solo foi observada na ��rea de mata. Entre os atributos do solo utilizados como indicadores de qualidade do solo os mais sens��veis aos n��veis de pastagens degradadas s��o os atributos qu��micos pH, Ca2+, Mg2+, Al3+, H+Al, satura����o por bases (V), satura����o por alum��nio (m), seguidos pelos atributos f��sicos macroporosidade e porosidade total. Os compartimentos da mat��ria org��nica do solo, mat��ria org��nica particulada (MOP), mat��ria org��nica leve (MOL) e carbono sol��vel em ��gua (CSA) utilizados como indicadores de qualidade do solo s��o eficientes em diferenciar a qualidade do solo nas convers��es de sistema, mata/pastagens e pastagens/capoeiras, n��o sendo sens��vel aos n��veis de pastagens degradadas, o que sugere estudos futuros utilizado compartimentos mais sens��veis a pequenas varia����es. As determina����es de campo espessura do horizonte ���A���, profundidade do sistema radicular e taxa de cobertura do solo s��o sens��veis aos n��veis de pastagens degradadas, e apresentam uma boa correla����o com os indicadores de laborat��rio macroporosidade (Ma), mat��ria org��nica particulada (MOP), satura����o por bases (V), satura����o por alum��nio (m) sugerindo assim a utiliza����o dessas determina����es como indicadores de qualidade do solo em pastagens degradadas, para o solo e a regi��o estudados.