Simultaneous monitoring of electrical and secretory activity in peptidergic neurosecretory terminals of the crab.

Intracellularly recorded responses of peptidergic neurosecretory terminals and somata were correlated with their secretory responsiveness to elevation of the external K concentration ([K+]o). The experiments were performed on in vitro X-organ sinus gland neurosecretory systems from the eyestalk of the crab Cardisoma carnifex. Elevated-K-evoked release was followed in preparations exposed to a pulse-chase radiolabelling regime. The release of [3H]leucine incorporated into neurosecretory peptides could be followed by collecting the separate perfusates of the somata and terminal regions. Elevation of the [K+]o evoked terminal depolarization, an increase in impulse firing frequency and a decrease (50%) in terminal input resistance. Impulse firing ceased (in ca. 2 min) as depolarization reached a sustained maximum level (-17.6 +/- 3.57 mV, n = 9, absolute potential). The terminal depolarization and decreased input resistance were maintained throughout the period of elevated-K treatment. Release from the terminal region of incorporated 3H label paralleled the simultaneously monitored terminal depolarization. Maintained exposure to elevated-K saline was accompanied by sustained high levels of 3H release continuing beyond the loss of regenerative membrane responses. Release declined with a half-time of 47.1 +/- 13.5 min (n = 7). In contrast, terminal release of red pigment concentrating hormone (RPCH) was transitory, reaching peak values and declining to base line within a 10 min period. Removal of external Ca or addition of the Ca antagonists, Cd or Mn, blocked the stimulated 3H release. Addition of Cd or Mn, prior to or during an elevated-K-evoked 3H release produced a reversible suppression of the secretory response. Stimulation in the absence of external Na, under normal Ca conditions, resulted in a normal secretory response. The amplitude and duration of the elevated-K-evoked terminal depolarization was unaffected by nominally Ca- or Na-free saline or addition of Cd. Cd (1mM) and Na-free saline were effective in removing a Ca and Na component, respectively, of spontaneous or evoked terminal action potentials. Somatic responses to direct application of elevated K exhibited membrane depolarization and an accompanying increase in impulse firing. In contrast to recordings from the terminals in elevated K, fast regenerative potentials, electrotonically conducted from the distal axon, persisted in the somatic records. Somatic secretion of RPCH was below detectable limits (less than 0.2 fmol min-1). 3H release was an order of magnitude less than from the terminal region under similar conditions.(ABSTRACT TRUNCATED AT 400 WORDS)