Calcium in Individual Nerve Terminals from the Neurohypophysis

The release of transmitter/hormone from nerve terminals is dependent on depolar-ization-induced changes in intraterminal free [Ca2+], yet anatomical limitations of vertebrate nerve endings have largely precluded direct monitoring of such changes at single axon endings. As a result the quantitative relationship of the change in [Ca2+], to exocytosis in vertebrate endings remains virtually uncharacterized. Using axon terminals isolated from the neurohypophysis of the rat, changes in [Ca’+], in individual endings in response to depolarizing stimuli were directly quantitated by dual wavelength microspectrofluorornetry of cytoplasmic fura-2. The mean basal [Ca”]], was 66 & 4 nM (n = 212, kSEM), a value similar to most excitable and nonexcitable cells. Membrane depolarization evoked by elevation of extracellular [K‘] resulted in a rapid, dose-dependent increase in [Ca’+], that was not present in medium lacking extracellular Ca2+ (no added Ca2+ plus 1 mM EGTA) and was greatly reduced by the inorganic Ca2+ channel blockers Cd2+ and La3+. Application of the dihydropyridine nicardipine dose dependently reduced the rise in [Ca ” I], evoked by 50 mM K+ (93 % block at 10 wM) as did desmethoxyverapamil (D888). Thes