Caffeine activates a Ca(2+)-permeable, nonselective cation channel in smooth muscle cells

The effects of caffeine on cytoplasmic [Ca2+] ([Ca2+]i) and plasma membrane currents were studied in single gastric smooth muscle cells dissociated from the toad, Bufo marinus. Experiments were carried out using Fura-2 for measuring [Ca2+]i and tight-seal voltage-clamp techniques 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 ryanodine 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 induced by caffeine; moreover, the change in [Ca2+]i that did occur was tightly 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 caffeine. 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 suggesting 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 voltage-gated Ca2+ channels or when Ca2+ was released from carbachol-sensitive internal Ca2+ stores, the 80-pS channel was not activated. Moreover, caffeine was able to activate the channel in the presence of ryanodine at both positive and negative potentials, both conditions preventing release of Ca2+ from stores and the former preventing its influx. In summary, in gastric smooth muscle cells caffeine transiently releases 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.