Voltage-dependent calcium currents and cytosolic calcium in equine airway myocytes

1. The relationship between voltage-dependent calcium channel current (I-Ca) and cytosolic free calcium concentration ([Ca2+](i)) was studied in fura-2 AM-loaded equine tracheal myocytes at 35 degrees C and 1.8 mM Ca2+ using the nystatin patch clamp method. The average cytosolic calcium buffering constant was 77+/-3 (n=14), and the endogenous calcium buffering constant component is likely to be between 15 and 50. 2. I-Ca did not evoke significant calcium-induced calcium release (CICR) since (i) [Ca2+](i) scaled with the integrated I-Ca over the full voltage range of evoked calcium currents, (ii) increases in [Ca2+](i) associated with I-Ca were consistent with cytoplasmic buffering of calcium ions entering through voltage-dependent calcium channels (VDCCs) only (iii) there was a fixed instantaneous relationship between transmembrane calcium flux (J(Ca)) and the change in cytosolic free calcium concentration (Delta[Ca2+](i)) during I-Ca (iv) caffeine (8 mM) triggered 8-fold higher calcium transients than I-Ca, and (v) I-Ca evoked following release of intracellular calcium by caffeine resulted in an equivalent Delta[Ca2+](i)-J(Ca) relationship. 3. The time constant (tau) for the decay in [Ca2+](i) was 8.6+/-1.5 s (n=8) for single steps and 8.6+/-1.1 s (n=13) following multiple steps that increased [Ca2+](i) to much higher levels. Following application of caffeine (8 mM), however, [Ca2+](i) decay was enhanced (tau=2.0+/-0.2 s, n=3). The rate of [Ca2+](i) decay was not voltage dependent, was not decreased in the absence of extracellular Na+ ions, and no pump current was detected. 4. We conclude that under near physiological conditions, neither CICR nor Na+-Ca2+ exchange play a substantial role in the regulation of I-Ca-induced increases in [Ca2+](i), and that, even following release of intracellular calcium by caffeine, Na+-Ca2+ exchange does not play an appreciable role in the removal of calcium ions from the cytosol.