K+-dependence of Na+-Ca2+ exchange in type I vestibular sensory cells of guinea-pig

The properties of the vestibular Na+-Ca2+ exchanger in mammalian type I ves tibular sensory cells were studied using fura-2 fluorescence and immunocyto chemical techniques. In the absence of external Na+, the activation of Na+- Ca2+ exchange in reverse mode required the presence of external K+ (K-o(+)) and depended on K-o(+) concentration. Alkali cations Rb+ and NH4+ but not Li+ or Cs+ substituted for K-o(+) to activate the exchange. For pressure ap plications of 10 mM K-o(+) the contribution of voltage-sensitive calcium ch annels to the increase in [Ca2+](i) was <15%. The dependence of the exchang e on [K+](o) was also recorded when the membrane potential was clamped usin g carbonyl cyanide p-trifluoromethoxy-phenylhydrazone (FCCP) and monensin i onophores. In these conditions, where there was no intracellular Na+, the i ncrease in [Ca2+](i) was completely blocked, These physiological results su ggest that in reverse mode, Ca2+ entry is driven by both an outward transpo rt of Na+ acid an inward transport of K+, The dependence of the vestibular Na+-Ca2+ exchanger on K+ is more reminiscent of the properties of the retin al type Na+-Ca2+ exchanger than those of the more widely distributed cardia c type exchanger. Moreover, the immunocytochemical localization of both typ es of exchange proteins in the vestibular sensory epithelium confirmed the presence in the vestibular sensory cells of a Na+-Ca2+ exchanger which is r ecognized by an antibody raised against retinal type and not by an antibody raised against the cardiac type.