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A single glass micropipette voltage-clamp technique was used to study a potential-dependent calcium inward current in isolated smooth muscle cells of the guinea-pig taenia caeci. Experiments were performed at 22-24 degrees C. With potassium as the main cation in the pipette solution, a transient inward current appeared in response to a depolarizing pulse, followed by an outward current. The replacement of potassium ions by caesium ions and TEA (tetraethyl-ammonium) in the pipette solution resulted in an effective suppression of potassium outward current permitting a study of the calcium current solely. The calcium inward current was blocked by 5 mM-cobalt and 5 X 10(-6) M-verapamil. Activation of the calcium current occurred at a membrane potential of between -35 and -25 mV. The calcium current was maximal in the potential range +10 to +20 mV and did not reverse even at +60 mV. Inactivation of the calcium current had a complex nature. It did not inactivate completely even during depolarizations lasting many seconds. During the first 400 ms the decay of the calcium current followed a time course described by two exponentials. The fast time constant of decay was in the range of 40 to 53 ms (n = 3) and the slow time constant was approximately 10-fold greater (at 0 mV). The fast time constant did not depend on the membrane potential while the slow time constant decreased with depolarization. Availability of the calcium current was estimated in double-pulse experiments. It had a U-shaped dependence on the conditioning potential; maximal inactivation was observed at potentials corresponding to the maximal calcium current. It was suggested that a component of inactivation was dependent on the calcium current which flowed. Calculations of calcium entry at various depolarizations showed that large amounts of calcium ions enter the cell. Also, it was suggested that calcium ions are effectively bound within the smooth muscle cell.

SUMMARY1. Calcium current (ICa) was studied in single isolated smooth muscle cells of a guinea-pig taenia caeci dialysed with Cs'-containing solution to suppress K+ outward current.2. With increasing step depolarizations up to + 10 mV, acceleration of ICa inactivation was observed. With further increase of step depolarization, ICa inactivation was slowed down. The largest ICa (observed at + 10 mV) was characterized by the maximal speed of inactivation. 4. Elevation of temperature increased ICa peak amplitude and accelerated its decay. The amplitude of Ic. was increased by a factor of 1-7 +0-14 (n = 6) when the temperature was raised by 10 'C.

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SUMMARY1. The effects of Na-free and K-free solutions, tetraethyl ammonium (TEA), Mn2 , verapamil and ouabain on the electrophysiological properties ofthe smooth muscle cells of guinea-pig ureter have been studied, using the double sucrose-gap method.2. TEA (5 mM) increased the amplitude and duration of both the initial spike component and the subsequent plateau of the action potential. The repetitive spike discharge on the plateau was abolished. The amplitude and duration of the phasic contraction was increased. The threshold for excitation was lowered while the resting potential and membrane resistance were unaffected.3. In Na-free solution the duration of the action potential decreased, mainly due to the suppression of the plateau. A similar effect was produced by exposure to K-free solution and also by ouabain.4. Mn2+ (2 mM) suppressed the spike component and raised the threshold for excitation. The amplitude of the remaining part of the action potential was markedly increased but the contraction was rapidly abolished. The resting potential and membrane resistance were unchanged.When Mn2+ was added to Na-free solution it produced an increase in the amplitude and duration of the remaining part of the action potential but the phasic contraction was abolished.5. Verapamil did not specifically block the fast component of the action potential but initially increased the amplitude of the spike and shortened the plateau. Subsequently, both the action potential and the phasic contraction became smaller. by the initial spike component of the action potential, whereas the plateau is associated with the amplitude and particularly the duration of the contraction.