Effects of putative activators of K⁺ channels in mouse pancreatic β‐cells

Abstract

1 The vasodilator and antihypertensive properties of pinacidil, cromakalim (BRL 34915), nicorandil and minoxidil sulphate may be due, at least in part, to their ability to open K+ channels in vascular smooth muscles. In this study, mouse pancreatic islets were used to determine whether these drugs affect insulin release by acting on K + channels of fl-cells. Their effects were compared to those of diazoxide. 2 Diazoxide caused a dose-dependent inhibition of insulin release by islets incubated with 15mM glucose (93% at 1OOM). Pinacidil inhibited release by 36 and 72% at 100 and 500 M, respectively.Cromakalim and nicorandil were less effective (35 and 25% inhibition at 50OpM). Minoxidil sulphate increased insulin release at 500 pM. 3 In the presence of 7mm glucose and in the absence of Ca2+ (to avoid activation of Ca2+-dependent K+ channels), 86Rb efflux from islet cells was increased by 100-50OM pinacidil and 5OM nicorandil, which were, however, less potent than diazoxide. Cromakalim was ineffective, whereas 5OM minoxidil sulphate decreased the efflux rate. In the absence of glucose and presence of Ca2", 500 JIM cromakalim and minoxidil sulphate inhibited 86Rb efflux. 5 ATP-sensitive K+ currents were studied in single fl-cells by the whole cell patch-clamp technique. Pinacidil increased the current less than did diazoxide. In contrast, cromakalim and minoxidil sulphate decreased K+-currents whilst nicorandil was without effect. 6 It is concluded that pinacidil, like diazoxide, inhibits insulin release from fl-cells by opening ATP-sensitive K+ channels, whereas the smaller inhibitory effects of cromakalim and nicorandil may involve actions other than on K+ channels in these cells. Minoxidil sulphate potentiates glucose-induced insulin release, probably by inhibiting ATP-sensitive K+ channels. However, all these effects of the vasodilators are only seen at high concentrations and are thus unlikely to occur in vivo.

Effects of cromakalim on the membrane potassium permeability of frog skeletal muscle in vitro

“…potent in smooth muscle where the EC50 for the increase in 86Rb efflux is around 1 JM (Quast & Cook, 1989). In cardiac muscle the EC50 is in the range 5-30 JM (Osterrieder, 1988;Escande et al, 1988;Sanguinetti et al, 1988) whilst in insulin-secreting cells cromakalim is relatively ineffective (Garrino et al, 1989;Dunne et al, 1990). Our results in frog skeletal muscle, like those of Spuler et al (1989) in human muscle suggest a potency between those in cardiac muscle and insulin-secreting cells.…”

Clonidine inhibits ATP‐sensitive K+channels in mouse pancreatic β‐cells

“…Clonidine Effects of clonidine and adrenaline on A TP-sensitive K + currents At physiological ATP concentrations, the whole cell ATPsensitive K+ current is small because most of the channels are inhibited (Rorsman & Trube, 1985). When the cell is dialysed with a pipette solution containing a lower than physiological ATP concentration, ATP-sensitive K+ currents activate with time following the establishment of the whole cell configuration and then decline at a rate dependent on the ATP concentration (Trube et al, 1986;Garrino et al, 1989). Drugs were applied during the approximately linear phase of rundown after the current had attained a maximum.…”

“…After isolation of the islets, islet cells were dispersed and cultured for 1-2 days as previously described (Plant, 1988 (Garrino et al, 1989;Plant & Henquin, 1990 (Horn & Marty, 1988 (Smith et al, 1989). The tip of the pipette was first filled by dipping the pipette briefly in this solution, then back-filled with the same solution supplemented with 100 or 150 pg ml-1 nystatin in 0.2% dimethylsulphoxide (DMSO).…”

“…A wide variety of chemically-unrelated substances stimulate insulin secretion by closing adenosine 5'-triphosphate (ATP)-sensitive K + channels in pancreatic fl-cells whereas others inhibit secretion by opening these channels (Garrino et al, 1989;Cook & Quast, 1990;Henquin, 1990).…”

Effect of MgATP on pinacidil‐induced displacement of glibenclamide from the sulphonylurea receptor in a pancreatic β‐cell line and rat cerebral cortex

“…In pancreatic P-cells, the potency of diazoxide for activation of K+-channels and inhibition of insulin release is about 10 fold higher than the potency of pinacidil (Garrino et al, 1989 The total concentration of cytosolic ATP in islet cells ranges above 0.5 mM (Malaisse & Sener, 1987) and more than 90% of this concentration probably exists as MgATP complex (Corkey et al, 1986). Therefore, the states of the binding sites for pinacidil and sulphonylureas observed in the presence of MgATP might predominate in the intact P-cell.…”