Ionic dependence of secretory and electrical activity evoked by elevated K+ in a peptidergic neurosecretory system

Secretion of the octapeptide erythrophore- (red pigment-) concentrating hormone (ECH, RPCH) and extracellularly monitored electrical activity were followed simultaneously from individual, isolated sinus glands (neurohaemal organs), of the crab Cardisoma carnifex. Following introduction of saline having elevated [K], 100-196 mmol l-1 (5-11 X normal), secretion (bioassayed from 1-min fractions during continuous perfusion) increases from barely detectable (less than 1 fmol min-1) to a peak, average 31 fmol min-1, within 5 min, and immediately subsides. Additional responses are obtainable following a period, greater than 30 min, of normal saline perfusion. Secretory responses to K are Ca-dependent. If Ca is restored (in high K) following perfusion in 0-Ca, high K, only a small secretory response is observed. Addition of Mn (10 mmol l-1, normal Ca) reduces secretion to one-tenth. Increased net uptake of 45Ca of 2.5- to 6-fold is observed in individual sinus glands exposed to 10 X K compared to paired, unstimulated organs. The pattern and Ca-dependence of secretory responses to K are unaffected, but the amount of secretion is augmented in Na-deficient or TTX-containing salines. Intracellular recording confirms that brief (10-40 s) bouts of intense firing recorded extracellularly upon commencing a high K perfusion include repetitive firing by terminals, superimposed on rapid depolarization. Firing ceases as the membrane potential reaches a depolarized value (-18 to -15 mV for [K] 100-176 mmol l-1), which is then maintained until restoration of normal saline, when slow repolarization ensues. In 0-Ca, spontaneous impulse firing is increased, resting potential depolarized by 5 to 15 mV, but the bout of impulse firing and the maintained depolarization in response to K are similar. Thus, mechanisms of secretion of a crustacean peptide neurohormone appear closely similar to those of other systems characterized: responsiveness to elevated K, dependence on Ca, depolarization-, but not secretion-dependent inactivation, and lack of dependence on Na inward current. Intracellular recording here permits direct observation of electrical responses of terminals.