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

Abstract

1 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 Ca2c hannel current evoked by a depolarizing pulse (O 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 Ca2+-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 a,-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. Voltagesensitive Ca2+ channels are inactivated through an increase in intracellular Ca2+ induced by either activation of purinoceptor or muscarinic receptors.

Age-related differential susceptibility to calcium channel blocker and low calcium medium in rat detrusor muscle: Response to field stimulation

“…The results of both the diltiazem and low calcium studies support the concept that the cholinergic response involves both calcium influx and intracellular calcium release, whereas the purinergic response is mediated primarily if not exclusively by calcium influx. This concept is supported by a recent study performed on guinea pig urinary bladder by Nakayama [1993]. By using an intracellular recording technique, Nakayama [ 19931 has concluded that purinergic stimulation is mediated by the opening of non-selective cation channels.…”

Involvement of ryanodine receptors in muscarinic receptor-mediated membrane current oscillation in urinary bladder smooth muscle

“…In detrusor smooth muscle, muscarinic agonists are known to release Ca 2ϩ from the intracellular stores via G protein activation and subsequent IP 3 formation (26,36). Figure 5 shows the effects of the IP 3 receptor antagonists heparin and xestospongin C (cell membrane-impermeable and -permeable IP 3 receptor antagonists, respectively) (9, 32).…”

Purinergic signalling in the urinary tract in health and disease

“…Experiments on cultured smooth muscle cells from rabbit bladder, performed with the fura-2 technique to measure changes in intracellular Ca 2+ , showed that ATP produced a rapid but transient increase in [Ca 2+ ] i , while ACh produced a delayed, prolonged increase [534]. Studies on isolated smooth muscles cells from guinea-pig bladder, using a whole-cell voltage-clamp technique, confirmed that purinergic receptor stimulation opens nonselective cation channels, while muscarinic stimulation triggers Ca 2+ release from intracellular stores [512]. Different types of Ca 2+ channels are present prejunctionally and postjunctionally in the urinary bladder.…”