Changes in intracellular Ca2+concentration induced by L-type Ca2+ channel current in guinea pig gastric myocytes.

We investigated the relationship between voltage-operated Ca2+ channel current and the corresponding intracellular Ca2+concentration ([Ca2+]i) change (Ca2+ transient) in guinea pig gastric myocytes. Fluorescence microspectroscopy was combined with conventional whole cell patch-clamp technique, and fura 2 (80 μM) was added to CsCl-rich pipette solution. Step depolarization to 0 mV induced inward Ca2+ current ( I Ca) and concomitantly raised [Ca2+]i. Both responses were suppressed by nicardipine, an L-type Ca2+ channel blocker, and the voltage dependence of Ca2+transient was similar to the current-voltage relation of I Ca. When pulse duration was increased by up to 900 ms, peak Ca2+ transient increased and reached a steady state when stimulation was for longer. The calculated fast Ca2+ buffering capacity ( B value), determined as the ratio of the time integral of I Ca divided by the amplitude of Ca2+ transient, was not significantly increased after depletion of Ca2+ stores by the cyclic application of caffeine (10 mM) in the presence of ryanodine (4 μM). The addition of cyclopiazonic acid (CPA, 10 μM), a sarco(endo)plasmic reticulum Ca2+-ATPase inhibitor, decreased B value by ∼20% in a reversible manner. When KCl pipette solution was used, Ca2+-activated K+ current [ I K(Ca)] was also recorded during step depolarization. CPA sensitively suppressed the initial peak and oscillations of I K(Ca) with irregular effects on Ca2+transients. The above results suggest that, in guinea pig gastric myocyte, Ca2+ transient is tightly coupled to I Caduring depolarization, and global [Ca2+]iis not significantly affected by Ca2+-induced Ca2+ release from sarcoplasmic reticulum during depolarization.