Ca(2+) as second messenger in PTTH-stimulated prothoracic glands of the silkworm, Bombyx mori

Measurements of Ca(2+) influx and [Ca(2+)](i) changes in Fura-2/AM-loaded prothoracic glands (PGs) of the silkworm, Bombyx mori, were used to identify Ca(2+) as the actual second messenger of the prothoracicotropic hormone (PTTH) of this insect. Dose-dependent increases of [Ca(2+)](i) in PG cells were recorded in the presence of recombinant PTTH (rPTTH) within 5 minutes. The rPTTH-mediated increases of [Ca(2+)](i) levels were dependent on extracellular Ca(2+). They were not blocked by the dihydropyridine derivative, nitrendipine, an antagonist of high-voltage-activated (HVA) Ca(2+) channels, and by bepridil, an antagonist of low-voltage-activated (LVA) Ca(2+) channels. The trivalent cation La(3+), a non-specific blocker of plasma membrane Ca(2+) channels, eliminated the rPTTH-stimulated increase of [Ca(2+)](i) levels in PG cells and so did amiloride, an inhibitor of T-type Ca(2+) channels. Incubation of PG cells with thapsigargin resulted in an increase of [Ca(2+)](i) levels, which was also dependent on extracellular Ca(2+) and was quenched by amiloride, suggesting the existence of store-operated plasma membrane Ca(2+) channels, which can also be inhibited by amiloride. Thapsigargin and rPTTH did not operate independently in stimulating increases of [Ca(2+)](i) levels and one agent's mediated increase of [Ca(2+)](i) was eliminated in the presence of the other. TMB-8, an inhibitor of intracellular Ca(2+) release from inositol 1,4,5 trisphosphate (IP(3))-sensitive Ca(2+) stores, blocked the rPTTH-stimulated increases of [Ca(2+)](i) levels, suggesting an involvement of IP(3) in the initiation of the rPTTH signaling cascade, whereas ryanodine did not influence the rPTTH-stimulated increases of [Ca(2+)](i) levels. The combined results indicate the presence of a cross-talk mechanism between the [Ca(2+)](i) levels, filling state of IP(3)-sensitive intracellular Ca(2+) stores and the PTTH-receptor's-mediated Ca(2+) influx.