Abstract
Measurements of Ca^2+^ influx in Fura‐2/AM loaded steroidogenic cells (prothoracic glands; PGs) of the silkworm, Bombyx mori showed that application of the neuropeptide prothoracicotropic hormone (PTTH) can increase the intracellular [Ca^2+^]~i~. This PTTH‐mediated Ca^2+^ influx in PG cells had kinetic patterns and pharmacological characteristics similar to those induced by thapsigargin. Namely, it produced increases in intracellular Ca^2+^ levels only in the presence of extracellular Ca^2+^, it was blocked by Gd^3+^ and 2‐Aminoethoxydiphenylborate (2‐APB), and it was unaffected by several toxins or compounds that block voltage‐activated Ca^2+^ channels. Moreover, the PTTH‐stimulated increase of Ca^2+^ levels was eliminated in the presence of heparin (an IP~3~ receptor blocker), and by TMB‐8 which also blocked any PTTH‐dependent increase of ecdysteroid secretion. The PTTH‐mediated increase of Ca^2+^ levels was not affected by the non‐hydrolysable GDP analogue, GDPβS, an indication that a G protein is not downstream of the PTTH receptor. These results argue strongly in favor of gating by the PTTH receptor of capacitative Ca^2+^ entry (CCE) channels (or store‐operated Ca^2+^ channels (SOCs)) by a mechanism that does not involve any G proteins but requires the presence of functional IP~3~ receptors. Because the ability of PTTH to stimulate the [Ca^2+^]~i~ levels of PG cells was completely mimicked by thapsigargin and exhibited a pharmacological profile similar to CCE mechanisms, we believe that PTTH directly regulates a CCE pathway in PG cells thereby activating a plethora of downstream regulators responsible for ecdysteroid secretion by the PGs of Bombyx mori. J. Exp. Zool. 303A:101–112, 2005. © 2005 Wiley‐Liss, Inc.