ACETYLCHOLINE CA2+ STORES REFILLING DIRECTLY INVOLVES A DIHYDROPYRIDINE-SENSITIVE CHANNEL IN DOG TRACHEA

Repetitive stimulation of the smooth muscle with acetylcholine (ACh) in the continuous presence of nifedipine resulted in a progressive decrease in the developed tension. This was associated with a decrease in the content of the agonist-sensitive intracellular Ca2+ stores. Agonist-sensitive internal Ca2+ stores appeared to be readily depleted by successive or prolonged agonist stimulation in Ca2+-free medium. The refilling of the empty stores when the muscle is at rest required extracellular Ca2+, was decreased by nifedipine, and was increased by BAY K 8644 and by increased external Ca2+ concentration. Refilling of stores during ACh stimulation in Ca2+-containing medium was decreased by nifedipine and by cyclopiazonic acid (CPA), an inhibitor of the sarcoplasmic reticulum (SR) Ca2+ pump, and was potentiated by BAY K 8644. BAY K 8644 reversed the inhibitory effect of CPA on stores Ca2+ refilling. Ryanodine in normal Krebs increased muscle resting tension, an effect not observed in Ca2+-free medium, blocked by nifedipine and enhanced by BAY K 8644. We propose that the refilling of ACh-sensitive internal Ca2+ stores involves two distinct pathways, one dependent on the uptake of cytosolic Ca2+ via a CPA-sensitive SR Ca2+-adenosinetriphosphatase, and the other pathway dependent on extracellular Ca2+ influx via a dihydropyridine-sensitive Ca2+ channel and is CPA insensitive. The refilling pathway between plasmalemma and SR may involve a plasmalemma L-type Ca2+ channel (dihydropyridine sensitive) and the SR Ca2+ release channel (ryanodine sensitive).