Figure 3.

Involvement of PKC activation in the stimulation of insulin secretion in HIT T15 cells by InsP₆. (A) Modulation of PKC activity by 50 μM InsP₆ (▪) in the absence and the presence of free Ca²⁺ as described in Materials and Methods. (□) The enzyme activity without InsP₆. Data represent mean ± SEM of four experiments. ∗, P < 0.05, relative to PKC activity in the absence of free Ca²⁺ and InsP₆. (B) Inhibitor of PKC, 1.5 μM Calphostin C (Calph. C), blocked increase in insulin secretion induced by 50 μM InsP₆. (C) Requirement of ATP-hydrolysis for the development of the stimulatory effect of 50 μM InsP₆ on insulin secretion. ATP (2 mM) was substituted by 2 mM AMP-PCP, a nonhydrolyzable analog of ATP. (D) Additive effect of 50 μM InsP₆ and 1 μM okadaic acid (OA) on stimulation of insulin secretion and the absence of additive effect of 50 μM InsP₆ and 100 nM TPA. In B–D insulin secretion was measured in buffer with basal Ca²⁺ concentration (30 nM). (□) Insulin secretion without InsP₆ and black columns illustrate insulin secretion with 50 μM InsP₆. The presence of other compounds is indicated in the figure. For measurements of insulin secretion, data represent mean ± SEM for 17 (B), 18 (C), and 20 (D) observations from three separate experiments. ∗∗∗, P < 0.001, relative to insulin secretion at 30 nM Ca²⁺. ###, P < 0.001, relative to insulin secretion in the presence of 50 μM InsP₆. §§§, P < 0.001, relative to insulin secretion in the presence of 1 μM OA.