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 (I-caf) 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 I-caf 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 (I-Cl-Ca). When 10 mM caffeine was puff-applied twice with various intervals, the amplitude of the second I-caf depended upon the period of the interval. The relationship between the amplitude and the interval represents the recovery time course of I-caf, which was significantly slowed by 30 mu M cyclopiazonic acid. The I-caf was not significantly affected by addition of Cd2+. Removal of external Ca2+ did not affect the first I-caf but markedly reduced the second one, regardless of the presence of Cd2+. In conclusion, I-caf is evoked by activation of I-Cl-CA via Ca2+ release. The recovery time course of I-caf 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 Cd2+-insensitive pathway. Spontaneous transient inward currents may be also due to I-Cl-Ca activated by spontaneous Ca2+ release from local storage sites.