Hyperpolarisation-activated inward current in isolated sheep mesenteric lymphatic smooth muscle.

1. Freshly isolated sheep lymphatic smooth muscle cells were studied using the perforated patch-clamp technique. Hyperpolarisation with constant-current pulses caused a time-dependent rectification evident as a depolarising 'sag' followed by an anode-break overshoot at the end of the pulse. Both sag and overshoot were blocked with 1 mM Cs+. 2. Cells were voltage clamped at -30 mV and stepped to -120 mV in 10 mV steps of 2 s duration. Steps negative to -60 mV evoked a slowly activating, non-inactivating inward current which increased in size and rate of activation with increasing hyperpolarisation. 3. The slowly activating current was reduced in Na+-free bathing solution but enhanced when the extracellular K+ concentration was increased to 60 mM. The current was significantly reduced by 1 mM Cs+ and 1 microM ZD7288 but not by 1.8 mM Ba2+. 4. The steady-state activation curve of the underlying conductance showed a threshold at -50 mV and half-maximal activation at -81 mV. Neither threshold nor half-maximal activation was significantly affected by increasing the external K+ concentration to 60 mM. 5. The frequency of spontaneous contractions and fluid propulsion in isolated cannulated segments of sheep mesenteric lymphatics were decreased by 1 mM Cs+ and by 1 microM ZD7288. 6. We conclude that sheep lymphatics have a hyperpolarisation-activated inward current similar to the If seen in sinoatrial node cells of the heart. Blockade of this current slows spontaneous pumping in intact lymphatic vessels suggesting that it is important in normal pacemaking.

Outward currents in smooth muscle cells isolated from sheep mesenteric lymphatics.

1. The patch-clamp technique was used to measure membrane currents in isolated smooth muscle cells dispersed from sheep mesenteric lymphatics. Depolarizing steps positive to -30 mV evoked rapid inward currents followed by noisy outward currents. 2. Nifedipine (1 microM) markedly reduced the outward current, while Bay K 8644 (1 microM) enhanced it. Up to 90% of the outward current was also blocked by iberiotoxin (Kd = 36 nM). 3. Large conductance (304 +/- 15 pS, 7 cells), Ca(2+)- and voltage-sensitive channels were observed during single-channel recordings on inside-out patches using symmetrical 140 mM K+ solutions (at 37 degrees C). The voltage required for half-maximal activation of the channels (V1/2) shifted in the hyperpolarizing direction by 146 mV per 10-fold increase in [Ca2+]i. 4. In whole-cell experiments a voltage-dependent outward current remained when the Ca(2+)-activated current was blocked with penitrem A (100 nM). This current activated at potentials positive to -20 mV and demonstrated the phenomenon of voltage-dependent inactivation (V1/2 = -41 +/- 2 mV, slope factor = 18 +/- 2 mV, 5 cells). 6. Tetraethylammonium (TEA; 30 mM) reduced the voltage-dependent current by 75% (Kd = 3.3 mM, 5 cells) while a maximal concentration of 4-aminopyridine (4-AP; 10 mM) blocked only 40% of the current. TEA alone had as much effect as TEA and 4-AP together, suggesting that there are at least two components to the voltage-sensitive K+ current. 7. These results suggest that lymphatic smooth muscle cells generate a Ca(2+)-activated current, largely mediated by large conductance Ca(2+)-activated K+ channels, and several components of voltage-dependent outward current which resemble 'delayed rectifier' currents in other smooth muscle preparations.

Effect of purinergic blockers on outward current in isolated smooth muscle cells of the sheep bladder

Freshly dispersed cells from sheep urinary bladder were voltage clamped using the whole cell and inside-out patch-clamp technique. Cibacron and Basilen blue increased outward current in a dose-dependent manner with a half-maximal response at 10(-5) M. Suramin, in concentrations to 10(-3) M, had no such effect. The Cibacron blue response was abolished in Ca2+-free physiological salt solution, suggesting that it was acting on a Ca2+-dependent current. Similarly, the Cibacron blue-sensitive current was significantly attenuated by charybdotoxin. Cibacron blue did not modulate inward current nor were its effects modified by caffeine or heparin, suggesting that its effect on outward current was not secondary to an increase in intracellular Ca2+. Application of 10(-4) M Cibacron blue to the inside membrane of excised patches caused a rapid increase in open probability of a large-conductance (300 pS) K+ channel. These results suggest that Cibacron blue is a potent activator of a Ca2+-dependent outward current in bladder smooth muscle cells in addition to its action as a purinergic blocker.

Monitoring electrical resistance of concretes containing alternative cementitious materials to assess their resistance to chloride penetration

This is an invited contribution in a special issue of the Journal of Cement and Concrete Composites

Analysis and Synthesis of pHEMT Class-E Amplifiers with Shunt Inductor including ON-State Active-Device Resistance Effects

In this theoretical paper, the analysis of the effect that ON-state active-device resistance has on the performance of a Class-E tuned power amplifier using a shunt inductor topology is presented. The work is focused on the relatively unexplored area of design facilitation of Class-E tuned amplifiers where intrinsically low-output-capacitance monolithic microwave integrated circuit switching devices such as pseudomorphic high electron mobility transistors are used. In the paper, the switching voltage and current waveforms in the presence of ON-resistance are analyzed in order to provide insight into circuit properties such as RF output power, drain efficiency, and power-output capability. For a given amplifier specification, a design procedure is illustrated whereby it is possible to compute optimal circuit component values which account for prescribed switch resistance loss. Furthermore, insight into how ON-resistance affects transistor selection in terms of peak switch voltage and current requirements is described. Finally, a design example is given in order to validate the theoretical analysis against numerical simulation.