VOLTAGE-ACTIVATED POTASSIUM CURRENTS OF RABBIT OSTEOCLASTS - EFFECTS OF EXTRACELLULAR CALCIUM

The effects of increased extracellular Ca2+ concentration ([Ca2+](e)) were examined on a delayed-rectifier K+ current (I-K) and an inward-rectifier K+ current (I-K1) in rabbit osteoclasts. Elevation of [Ca2+](e) from 1.8 to 18 mM shifted the half point for I-K activation by +11.5 mV and the voltage dependence of inactivation by +9.7 mV and slowed the rate of I-K activation and deactivation. These effects of elevated [Ca2+](e) on I-K are consistent with screening of cell surface negative charge. However, elevation of [Ca2+](e) increased the voltage-dependent kinetics of I-K inactivation at all potentials tested, inconsistent with that predicted by simple surface charge theory. This finding suggests an additional, regulatory role for [Ca2+](e) in the gating of I-K channels. Some osteoclasts had an I-K1, which was decreased when [Ca2+](e) was raised from 1.8 to 18 mM. The physiological function of both types of K+ currents remains to be determined, and it is not clear whether these currents are involved with the coupling of cytosolic [Ca2+] to [Ca2+](e).