Characteristics of caffeine-induced and spontaneous inward currents and related intracellular Ca2+ storage sites in guinea-pig tracheal smooth muscle cells.

Characteristics of caffeine-induced inward current (Icaf) and spontaneous transient inward current were examined in single smooth muscle cells isolated from guinea-pig trachea. When a pipette solution contained mainly CsCl, an application of 10 mM caffeine elicited transient Icaf at a holding potential of -60 mV. Spontaneous transient inward currents were recorded in about 15% of cells examined and were abolished by caffeine. Both were Cl- current activated by an increase in intracellular Ca2+ concentration (ICl-Ca). When 10 mM caffeine was puff-applied twice with various intervals, the amplitude of the second Icaf depended upon the period of the interval. The relationship between the amplitude and the interval represents the recovery time course of Icaf, which was significantly slowed by 30 microM cyclopiazonic acid. The Icaf was not significantly affected by addition of Cd2+. Removal of external Ca2+ did not affect the first Icaf but markedly reduced the second one, regardless of the presence of Cd2+. In conclusion, Icaf is evoked by activation of ICl-Ca via Ca2+ release. The recovery time course of Icaf indicates the refilling of Ca2+ storage sites by the cyclopiazonic acid-sensitive Ca2+ pump. The refilling at -60 mV depends strongly upon Ca2+ influx through the Cd(2+)-insensitive pathway. Spontaneous transient inward currents may be also due to ICl-Ca activated by spontaneous Ca2+ release from local storage sites.