CAFFEINE ACTIVATES A CA2-PERMEABLE, NONSELECTIVE CATION CHANNEL IN SMOOTH-MUSCLE CELLS()

The effects of caffeine on cytoplasmic [Ca2+] ([Ca2+](i)) and plasma m embrane currents were studied in single gastric smooth muscle cells di ssociated from the toad, Bufo marinus. Experiments were carried out us ing Fura-2 for measuring [Ca2+](i) and tight-seal voltage-clamp techni ques for recording membrane currents. When the membrane potential was held at - 80 mV, in 15% of the cells studied caffeine increased [Ca2+] (i) without having any effect on membrane currents. In these cells rya nodine completely abolished any caffeine induced increase in [Ca2+](i) . In the other cells caffeine caused both an increase in [Ca2+](i) and activation of an 80-pS nonselective cation channel. In this group of cells ryanodine only partially blocked the increase in [Ca2+](i) induc ed by caffeine; moreover, the change in [Ca2+](i) that did occur was t ightly coupled to the time course and magnitude of the cation current through these channels. In the presence of ryanodine, blockade of the 80-pS channel by GdCl3 or decreasing the driving force for Ca2+ influx through the plasma membrane by holding the membrane potential at +60 mV almost completely blocked the increase in [Ca2+](i) induced by caff eine. Thus, the channel activated by caffeine appears to be permeable to Ca2+. Caffeine activated the cation channel even when [Ca2+](i) was clamped to below 10 nM when the patch pipette contained 10 mM BAPTA s uggesting that caffeine directly activates the channel and that it is not being activated by the increase in Ca2+ that occurs when caffeine is applied to the cell. Corroborating this suggestion were additional results showing that when the membrane was depolarized to activate vol tage-gated Ca2+ channels or when Ca2+ was released fr om carbachol-sen sitive internal Ca2+ stores, the 80-pS channel was not activated. More over, caffeine was able to activate the channel in the presence of rya nodine at both positive and negative potentials, both conditions preve nting release of Ca2+ from stores and the former preventing its influx . In summary, in gastric smooth muscle cells caffeine transiently rele ases Ca2+ from a ryanodine-sensitive internal store and also increases Ca2+ influx through the plasma membrane by activating an 80-pS cation channel by a mechanism which does not seem to involve an elevation of [Ca2+](i).