IONIC CURRENTS AND ENDOTHELIN SIGNALING IN SMOOTH-MUSCLE CELLS FROM RAT RENAL RESISTANCE ARTERIES

The repertoire of ionic channels expressed in myocytes freshly isolated from microdissected interlobar and arcuate arteries of rat kidney and their integrative behavior in response to endothelin-1 (ET-1) were studied by identification and characterization of major whole cell current components using patch-clamp technique. In renal microvascular smooth muscle cells (RMSMC) dialyzed with K+-containing solution, rapidly inactivating (I-to) and sustained outward K+ currents were identified. Voltage-dependent I-to was categorized as ''A'' current based on its kinetics, sensitivity to 4-aminopyridine (4-AP), and refractoriness to tetraethylammonium (TEA(+)). Ca2+-activated component of K+ current was completely blocked by 10 mM TEA(+), whereas 5 mM 4-AP did not affect this current. Maximal Ca2+ current (I-Ca) recorded in Cs+-loaded RMSMC reached 250 pA when cells were bathed in a solution with 2.5 mM Ca2+. Two patterns of I-Ca differing in kinetics, voltage range of activation and inactivation, and sensitivity to nifedipine were identified as T and L currents. Ca2+-dependent current component showing reversal potential near Cl- current (E(Cl)) and sensitivity to blocking action of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid was identified as Ca2+-activated E(Cl). Activation of RMSMC with ET-1 (1-10 nM) induced elevation of [Ca2+](i) and subsequent activation of Ca2+-activated I-Cl, which led to membrane depolarization sufficient to activate voltage-gated Ca2+ channels. ET-1-evoked transient reduction of I-Ca carried through voltagegated Ca2+ channels was followed by augmentation of L-type I-Ca. ET-1-induced mobilization of intracellular Ca2+, accompa nied by membrane depolarization, resulted in activation of Ca2+-dependent K+ channels, which can play the role of a feedback element terminating ET-1-induced membrane depolarization.