STIMULATION OF ECDYSTEROIDOGENESIS BY SMALL PROTHORACICOTROPIC HORMONE - ROLE OF CALCIUM

Insect prothoracic glands are regulated by neuropeptide prothoracicotr opic hormones (PTTH). In Manduca sexta PTTH exists as two size variant s, big PTTH (similar to 25.5 kDa) and small PTTH (similar to 7 kDa). P revious studies indicate that both size variants employ cAMP as a seco nd messenger and that stimulation of ecdysteroid secretion by big PTTH is Ca2+-dependent. In the present study, experiments were performed t o assess the role of Ca2+ in small PTTH-stimulated ecdysteroid secreti on by prothoracic glands from fifth instar larvae. Basal ecdysteroid s ecretion was not affected by Ca2+ channel blockers (verapamil or lanth anum) or by omission of Ca2+ from the incubation medium. Treatment of glands with a Ca2+ ionophore (A23187 or ionomycin) produced a concentr ation-dependent stimulation of ecdysteroid secretion. Stimulation of e cdysteroid secretion by small PTTH was suppressed (1) by Ca2+ channel blockers and (2) in Ca2+-free medium. A cAMP analog (Sp-cAMPS) stimula ted ecdysteroid secretion in the presence of a Ca2+ channel blocker (v erapamil) and in Ca2+-free incubation medium, and ionophore-induced ec dysteroid secretion appeared to be suppressed by a cAMP antagonist (Rp -cAMPS). The combined results indicate that basal ecdysteroid secretio n is not dependent on external Ca2+, and suggest that small PTTH-stimu lated ecdysteroid secretion is mediated by an influx of Ca2+ that prec edes cAMf formation.