The role of the L-type Ca(2+) channel in refilling functional intracellular Ca(2+) stores in guinea-pig detrusor smooth muscle.
The transient rise of intracellular Ca(2+) in detrusor smooth muscle cells is due to the release of Ca(2+) from intracellular stores. However, it is not known how store refilling is maintained at a constant level to ensure constancy of the contractile response. The aim of these experiments was to characterise the role of L-type Ca(2+) channels in refilling. Experiments used isolated guinea-pig detrusor myocytes and store Ca(2+) content was estimated by measuring the magnitude of change to the intracellular [Ca(2+)] ([Ca(2+)](i)) after application of caffeine or carbachol using epifluorescence microscopy. Membrane potential was controlled when necessary by voltage clamp. After Ca(2+) stores were emptied they refilled with an exponential time course, with a time constant of 88 s. The value of the time constant was similar to that of the undershoot of [Ca(2+)](i) following store Ca(2+) release. The degree of store filling was enhanced by maintained depolarisation, or by transient depolarising pulses, and attenuated by L-type Ca(2+) channel antagonists. Inhibition of the sarcoplasmic reticular Ca(2+)-ATPase prevented refilling. Reduction of the resting [Ca(2+)](i) was accompanied by membrane depolarisation; under voltage clamp reduction of [Ca(2+)](i) decreased the number and magnitude of spontaneous transient outward currents. Ca(2+) release from intracellular stores, elicited by caffeine or carbachol, is independent of membrane potential under physiological conditions. However, store refilling occurs via Ca(2+) influx through L-type Ca(2+) channels. Ca(2+) influx is regulated by a feedback mechanism whereby a fall of [Ca(2+)](i) reduces the activity of Ca(2+)-activated K(+) channels, causing cell depolarisation and an enhancement of L-type Ca(2+) channel conductance.
Wu
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Department of Medicine, University College London, 48 Riding House Street, London W1W 7EY, UK.
Sui
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Fry
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D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000502:physiology*; D000252:Calcium-Transporting ATPases / Q000378:metabolism; D002217:Carbachol / Q000494:pharmacology; D018679:Cholinergic Agonists / Q000494:pharmacology; D004594:Electrophysiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006168:Guinea Pigs; D007211:Indoles / Q000494:pharmacology; D007425:Intracellular Membranes / Q000378:metabolism* / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D009130:Muscle, Smooth / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum; D001743:Urinary Bladder / Q000378:metabolism*
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D018679:Cholinergic Agonists; D004791:Enzyme Inhibitors; D007211:Indoles; D002217:Carbachol; D000252:Calcium-Transporting ATPases; D002118:Calcium; C000543:cyclopiazonic acid