Phorbol Ester Impairs Electrical Excitation Of Rat Pancreatic Betacells Through Pkc-Independent Activation Of Katp Channels
Background: Phorbol 12-myristate 13-acetate (PMA) is often used as an activating phorbol ester of protein kinase C (PKC) to investigate the roles of the kinase in cellular functions. Accumulating lines of evidence indicate that in addition to activating PKC, PMA also produces some regulatory effects in a PKC-independent manner. In this study, we investigated the non-PKC effects of PMA on electrical excitability of rat pancreatic Î²-cells by using patch-clamp techniques. Results: In current-clamp recording, PMA (80 nM) reversibly inhibited 15 mM glucose-induced action potential spikes superimposed on a slow membrane depolarization and this inhibition can not be prevented by pre-treatment of the cell with a specific PKC inhibitor, bisindolylmaleimide (BIM, 1 Î¼M). In the presence of a subthreshold concentration (5.5 mM) of glucose, PMA hyperpolarized Î²-cells in a concentration-dependent manner (0.8-240 nM), even in the presence of BIM. Based on cell-attached single channel recordings, PMA increased ATP-sensitive K+ channel (KATP) activity. Based on inside-out patch-clamp recordings, PMA had little effect on KATP activity if no ATP was in the bath, while PMA restored KATP activity that was suppressed by 10 Î¼M ATP in the bath. In voltage-clamp recording, PMA enhanced tolbutamide-sensitive membrane currents elicited by repetitive ramp pulses from -90 to -50 mV in a concentration-dependent manner, and this potentiation could not be prevented by pre-treatment of cell with BIM. 4Î±-phorbol 12,13-didecanoate (4Î±-PDD), a non-PKC-activating phorbol ester, mimicked the effect of PMA on both current-clamp and voltage-clamp recording configurations. With either 5.5 or 16.6 mM glucose in the extracellular solution, PMA (80 nM) increased insulin secretion from rat islets. However, in islets pretreated with BIM (1 Î¼M), PMA did not increase, but rather reduced insulin secretion. Conclusion: In rat pancreatic Î²-cells, PMA modulates insulin secretion through a mixed mechanism: increases insulin secretion by activation of PKC, and meanwhile decrease insulin secretion by impairing Î²-cell excitability in a PKC-independent manner. The enhancement of KATP activity by reducing sensitivity of KATP to ATP seems to underlie the PMA-induced impairment of Î²-cells electrical excitation in response to glucose stimulation. Â© 2001 Suga et al; licensee BioMed Central Ltd.
Digital Object Identifier (DOI)
Suga, Sechiko; Wu, Jie; Ogawa, Yoshiji; Takeo, Teruko; Kanno, Takahiro; and Wakui, Makoto, "Phorbol Ester Impairs Electrical Excitation Of Rat Pancreatic Betacells Through Pkc-Independent Activation Of Katp Channels" (2001). Neurobiology. 460.