Inward current in single smooth muscle cells of the guinea pig taenia coli

Abstract

Using the tight-seal voltage-clamp method, the ionic currents in the enzymatically dispersed single smooth muscle cells of the guinea pig taenia coli have been studied. In a physiological medium containing 3 mM Ca2+ the cells are gently tapering spindles, averaging 201 (length) × 8 µm (largest diameter in center of cell), with a volume of 5 pl. The average cell capacitance is 50 pF, and the specific membrane capacitance 1.15 µF/cm2. The input impedance of the resting cell is 1-2 GΩ. Spatially uniform voltage-control prevails after the first 400 µs. There is much overlap of the inward and outward currents, but the inward current can be isolated by applying Cs+ internally to block all potassium currents. The inward current is carried by Ca2+ Activation begins at --30 mV, maximum Ica occurs at +10-+20 mV, and the reversal potential is -+75 inV. The Ca2+ channel is permeable to Sr2+ and Ba2+, and to Cs+ moving outwards, but not to Na+ moving inwards. Activation and deactivation are very rapid at ~33°C, with timeconstants of ≥ 1 ms. Inactivation has a complex time course, resolvable into three exponential components, with average time constants (at 0 mV) of 7, 45, and 400 ms, which are affected differently by voltage. Steady-state inactivation is half-maximal at -30 mV for all components combined, but -36 mV for the fast component and -26 and -23 mV for the other two components. The presence of multiple forms of Ca2+ channel is inferred from the inactivation characteristics, not from activation properties. Recovery of the fast channel occurs with a time-constant of 72 ms (at + 10 mV). Ca2+ influx during an action potential can transfer ~9 pC of charge, which could elevate intracellular Ca2+ concentration adequately for various physiological functions. © 1989, Rockefeller University Press., All rights reserved.