Regulation of exocytosis in electrically permeabilized insulin-secreting cells. Evidence for Ca²⁺ dependent and independent secretion

The regulation of insulin secretion from RINm5F cells exposed to high voltage discharge has been investigated. Electron microscopy revealed that the overall structure of the cells was preserved after permeabilization. In this preparation insulin release was stimulated by Ca²⁺ (EC₅₀=2.4 μM). The stable GTP analogue GTPγS enhanced secretion both at intermediate (nano- to micromolar) and vanishingly low (<10 pM) Ca²⁺ concentrations. At optimal Ca²⁺ (10 μM) the effect of GTPγS was greatly reduced. We investigated whether the secretory response to GTP analogues was mediated by any of three enzyme systems regulated by GTP-binding proteins, i.e. generation of cyclic AMP by adenylate cyclase, of diacylglycerol by phospholipase C and of arachidonic acid by phospholipase A₂. The involvement of these messenger systems could be excluded as (i) cyclic AMP only had minor, Ca²⁺ dependent effects, (ii) phospholipase C was not activated in the absence of Ca²⁺ and insulin secretion due to the phorbol ester TPA displayed a different Ca²⁺ dependency, (iii) arachidonic acid did not elicit Ca²⁺ independent insulin secretion. These results, taken together with the finding that insulin secretion due to Ca²⁺ or TPA is attenuated by the inhibitory guanine nucleotide GDPβS, suggest the existence of a regulatory site in exocytosis which is sensitive to guanine nucleotides.