Effects of excitatory neurotransmitters on Ca2+ channel current in smooth muscle cells isolated from guinea‐pig urinary bladder

A whole-cell voltage clamp technique was used to examine the effects of purinoceptor and muscarinic receptor agonists on voltage-sensitive Ca2+ channels in guinea-pig isolated urinary bladder cells. 2. When the cell membrane was clamped at the holding potential, rapid application of ATP elicited a large inward current in normal solution containing 2.5 mM Ca2+, and reduced the subsequent Ca2+ channel current evoked by a depolarizing pulse (0 mV). Carbachol (CCh) elicited little membrane current, but similarly reduced the Ca2+ current. 3. When purinoceptor agonists were rapidly applied during conditioning depolarizations at +80 mV, an outward current was elicited, and the Ca2+ channel current evoked by the subsequent test potential of 0 mV was not affected. Application of CCh at +80 mV also elicited an outward current, but it reduced the subsequently evoked Ca2+ current. 4. The inhibitory effect of muscarinic agonists on the Ca2+ channel current was attenuated by caffeine (10 mM). 5. In Ca(2+)-free, low-Mg2+ solution, a Na+ current flowing through voltage-dependent Ca2+ channels was evoked by depolarization. This current was not reduced by bath application of purinoceptor agonists (ATP and alpha,beta-methylene ATP). 6. These results suggest that the main effect of purinoceptor stimulation is opening of non-selective cation channels, and that muscarinic stimulation triggers Ca2+ release from intracellular stores. Voltage-sensitive Ca2+ channels are inactivated through an increase in intracellular Ca2+ induced by either activation of purinoceptor or muscarinic receptors.