ROLES OF CA2-POTENTIALS OF GUINEA-PIG URETERAL MYOCYTES( AND NA+ IN THE INWARD CURRENT AND ACTION)

Physiological roles of Ca2+ vs. Na+ in membrane currents and action po tentials of ureteral myocytes were investigated on freshly dissociated guinea pig ureteral myocytes with the patch-clamp method. The myocyte s are spindle shaped, with cell volume of 2,473 mu m(3), surface area of 2,014 mu m(2), capacitance of 48.2 pF, resting potential of -47.9 m V, and membrane conductance of 840 pS. The membrane current consists o f a slow inward Ca2+ current (I-Ca) conducted by L-type Ca2+ channels and an actively fluctuating Ca2+-activated K+ current [I-K(Ca)] conduc ted by Ca2+-activated maxi-K+ channels. I-Ca dominates the membrane cu rrent by being longlasting and more active at less depolarized potenti als than I-K(Ca) and by regulating I-K(Ca). Ca2+-free media, Co2+, and nifedipine reduce or block I-Ca, whereas high extracellular Ca2+ conc entration and BAY K 8644 enhance it. Action potential amplitudes and p lateaus are regulated correspondingly. Related changes are also seen i n I-K(Ca). in contrast, no fast inward current attributable to Na+ was found. Replacing extra cellular Na+ with tris(hydroxymethyl)aminometh ane had no apparent effects on the inward or outward current or on the action potentials.