Regulation of ecdysteroidogenesis in prothoracic glands of the tobacco hornworm Manduca sexta

Pharmacological study of signal transduction during stimulation of prothoracic glands from Manduca sexta

Cytosolic free calcium, [Ca(2+)](i), measured in individual prothoracic gland cells of Manduca sexta with Fura-2 was increased by prothoracicotropic hormone, PTTH, and by mastoparan, a wasp venom peptide, activating G proteins. The effect on [Ca(2+)](i) of mastoparan and of PTTH was inhibited by cadmium and the antagonist of T-type calcium channels, amiloride, and not influenced by the L-type calcium channel blocker nitrendipine, suggesting that the same or similar plasma membrane channels are involved in the action of mastoparan and of PTTH. Pertussis toxin prevented the mastoparan-induced increase of [Ca(2+)](i), whereas the effect of PTTH is not influenced by pertussis toxin. Intracellular addition of GDP-beta-S failed to inhibit the PTTH-stimulated increase in [Ca(2+)](i) suggesting that G proteins are not involved in the stimulatory mechanism of PTTH.

Intracellular calcium in prothoracic glands of Manduca sexta

Cytosolic free calcium was measured in individual prothoracic gland cells of Manduca larvae with Fura-2. During the last larval instar there was no correlation between intracellular calcium concentration and ecdysteroid secretion by the glands. The addition of prothoracicotropic hormone (PTTH) from brains of Manduca larvae to prothoracic glands in vitro resulted in a significant increase in the calcium concentration of the gland cells. The effect of PTTH was inhibited by the inorganic calcium channel antagonists, cadmium, lanthanum and nickel, and by the antagonist of T-type calcium channels, amiloride, whereas all the other antagonists tested failed to block the action of PTTH. TMB-8, an inhibitor of intracellular calcium mobilization, did not reduce the PTTH-induced rise in calcium, which suggests that IP(3)-dependent intracellular calcium stores are not involved in the calcium-mediated stimulation of ecdysteroid synthesis. Moreover, PTTH is thought to increase intracellular calcium in prothoracic glands of Manduca by influencing calcium channels in the plasma membrane.

Stimulation of ecdysteroidogenesis by small prothoracicotropic hormone: role of calcium

Insect prothoracic glands are regulated by neuropeptide prothoracicotropic hormones (PTTH). In Manduca sexta PTTH exists as two size variants, big PTTH (approximately 25.5 kDa) and small PTTH (approximately 7 kDa). Previous studies indicate that both size variants employ cAMP as a second messenger and that stimulation of ecdysteroid secretion by big PTTH is Ca(2+)-dependent. In the present study, experiments were performed to assess the role of Ca2+ in small PTTH-stimulated ecdysteroid secretion by prothoracic glands from fifth instar larvae. Basal ecdysteroid secretion was not affected by Ca2+ channel blockers (verapamil or lanthanum) or by omission of Ca2+ from the incubation medium. Treatment of glands with a Ca2+ ionophore (A23187 or ionomycin) produced a concentration-dependent stimulation of ecdysteroid secretion. Stimulation of ecdysteroid secretion by small PTTH was suppressed (1) by Ca2+ channel blockers and (2) in Ca(2+)-free medium. A cAMP analog (Sp-cAMPS) stimulated ecdysteroid secretion in the presence of a Ca2+ channel blocker (verapamil) and in Ca(2+)-free incubation medium, and ionophore-induced ecdysteroid secretion appeared to be suppressed by a cAMP antagonist (Rp-cAMPS). The combined results indicate that basal ecdysteroid secretion is not dependent on external Ca2+, and suggest that small PTTH-stimulated ecdysteroid secretion is mediated by an influx of Ca2+ that precedes cAMP formation.

Stimulation of ecdysteroidogenesis by small prothoracicotropic hormone: role of cyclic AMP

Prothoracicotropic hormones (PTTHs) stimulate synthesis and secretion of ecdysteroids by insect prothoracic glands. In Manduca sexta, PTTH exists as two size variants, small and big PTTH. Experiments were performed to assess the possible role of cyclic AMP in small PTTH signal transduction. cAMP analogs, or agents that increase intracellular cAMP, stimulated ecdysteroidogenesis. Small PTTH enhanced glandular cAMP levels; the rise in cAMP preceded an increase in ecdysteroid secretion. Prothoracic glands accumulated less cAMP when treated with small PTTH than when treated with big PTTH. A phosphodiesterase inhibitor (1-methyl-3-isobutylxanthine) (MIX) increased the amount of cAMP in glands treated with small but not big PTTH, suggesting that glandular phosphodiesterase activity may be elevated in the presence of small PTTH. PTTH-stimulated ecdysteroid secretion was suppressed by a cAMP antagonist (Rp-cAMPS). The effects of small and big PTTH on ecdysteroidogenesis were non-additive. The combined results suggest that cAMP is employed as a second messenger by both prothoracicotropins, and that there may be subtle differences in their respective mechanisms of action.