Ca2+/calmodulin inhibition and phospholipase C-linked Ca2+ Signaling in clonal beta-cells

Neurotransmitters and hormones, such as arginine vasopressin (AVP) and bombesin, evoke frequency-modulated repetitive Ca2+ transients in insulin-secreting HIT-T15 cells by binding to receptors linked to phospholipase C (PLC). The role of calmodulin (CaM)-dependent mechanisms in the generation of PLC-linked Ca2+ transients was investigated by use of the naphthalenesulfonamide CaM antagonists W-7 and W-13 and their dechlorinated control analogs W-5 and W-12. W-7 (10-30 microM) and W-13 (30-100 microM), but not W-5 (100 microM) and W-12 (300 microM), reversibly inhibited the AVP- and bombesin-induced Ca2+ transients. As the generation of PLC-linked Ca2+ transients requires mobilization of internal Ca2+ and Ca2+ influx through voltage-sensitive (VSCC) and -insensitive (VICC) Ca2+ channels, the effects of the W compounds on these processes were further investigated. First, W-7 dose dependently diminished K+ (45 mM)-induced Ca2+ signals (IC50, approximately 25 microM), and W-13 (100 microM) reduced the K+ (45 mM)-induced [Ca2+]i rise by about 40-60%, whereas W-5 (100 microM) and W-12 (300 microM) had no effect. In addition, W-7 (100 microM) inhibited whole cell Ca2+ currents in mouse beta-cells by about 60%. Second, pretreatment of cells (5 min) with W-7 (30 microM), but not W-5 (30 microM), inhibited agonist-induced internal Ca2+ mobilization by about 75% in Ca2+-free medium. Neither W-7 (30 microM) nor W-5 (30 microM) affected AVP (100 nM)-stimulated formation of IP3. Third, capacitative Ca2+ influx through VICC activated by thapsigargin (2 microM) in the presence of verapamil (50 microM) was inhibited by W-7 (30 microM) but not by W-5 (30 microM). As all of the W compound effects corresponded well to their reported anticalmodulin activity, a specific anticalmodulin action can be assumed. Thus, Ca2+ via activation of CaM-dependent processes could provide positive feedback on the generation of PLC-linked Ca2+ transients in HIT-T15 cells. This appears to involve CaM-dependent regulation of both mobilization of internal Ca2+ and Ca2+ influx through VSCC and VICC.