Pertussis toxin-sensitive pathway inhibits glucose-stimulated Ca2+ signals of rat islet beta-cells by affecting L-type Ca2+ channels and voltage-dependent K+ channels.
A role of pertussis toxin (PTX)-sensitive pathway in regulation of glucose-stimulated Ca2+ signaling in rat islet beta-cells was investigated by using clonidine as a selective agonist to alpha2-adrenoceptors which link to the pathway. An elevation of extracellular glucose concentration from 5.5 to 22.2 mM (glucose stimulation) increased the levels of [Ca2+]i of beta-cells, and clonidine reversibly reduced the elevated levels of [Ca2+]i. This clonidine effect was antagonized by yohimbine, and abolished in beta-cells pre-treated with PTX. Clonidine showed little effect on membrane currents including those through ATP-sensitive K+ channels induced by voltage ramps from -90 to -50 mV. Clonidine showed little effect on the magnitude of whole-cell currents through L-type Ca2+ channels (ICa(L)), but increased the inactivation process of the currents. Clonidine increased the magnitude of the voltage-dependent K+ currents (IVK). These clonidine effects on ICa(L) and IVK were abolished in beta-cells treated with PTX or GDP-betaS. These results suggest that the PTX-sensitive pathway increases IVK activity and decreases ICa(L) activity of islet beta-cells, resulting in a decrease in the levels of [Ca2+]i elevated by depolarization-induced Ca2+ entry. This mechanism seems responsible at least in part for well-known inhibitory action of PTX-sensitive pathway on glucose-stimulated insulin secretion from islet beta-cells.
Suga
Sechiko
S
Department of Physiology, Hirosaki University School of Medicine, 5 Zaifu-cho, Hirosaki 036-8562, Japan.
Takeo
Teruko
T
Nakano
Kyoko
K
Sato
Tatsusuke
T
Igarashi
Takanori
T
Yamana
Daisuke
D
Wakui
Makoto
M
D000818:Animals; D020746:Calcium Channels, L-Type / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D002478:Cells, Cultured; D003000:Clonidine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005947:Glucose / Q000378:metabolism*; D007515:Islets of Langerhans / Q000378:metabolism*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D037342:Pertussis Toxin / Q000494:pharmacology*; D024642:Potassium Channels, Voltage-Gated / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar
D020746:Calcium Channels, L-Type; D024642:Potassium Channels, Voltage-Gated; D037342:Pertussis Toxin; D005947:Glucose; D003000:Clonidine