Effects of 5-hydroxydecanoate on ATP-regulated potassium ion channels in insulin secreting cells

Abstract

910 2TN. 5-hydmxydecanoate (5-HD) is a component of hydmxyl fatty acids found in milk. Interest in 5-HD has recently been stimulated by the finding that the compound may in fact act as a novel antiarrhythmic agent, because of its blocking actions on ATP-regulated potassium ( K + A~) channels in cardiac myocytes [l]. K+ATP channels play an imponant role in governing the membrane potential of the insulin secreting cells of the pancreatic islets of Langerhans [see [2,3]), and we have therefore examined the actions of 5-HD in these cells using the techniques of patch-clamp electrophysiology and micmfluorimetry. Patch-clamp experiments were performed on the RINm5F insulinsecreting cells using cell-free inside-out patches and saponin 'opencells' under quasi-physiological cation gradients: 140mM Na+-rich solutions on the outside, and I 4 h M K+-rich solutions on the inside of the membrane. The effects were examined over a wide range of concentrations from lOnM to 500pkf. adding 5-HD direct to the inside face of the membrane alone, or in the presence of ATP (0.5mM). ADP (1 and 0.lmM) or ATP (0.5mM) + ADP (0.5mM). Changes in the cytosolic Ca2+ concentration ([Ca2+]i ) were assessed in single isolated cells through microfluonmetry techniques with Fura-2 loaded cells.In marked contrast to the effects of 5-HD on K + A~ channels in the heart. we find that in p-cells. 5-HD has much more complex actions. Not only was 5-HD able to induce channel block (n=15), but it was also able to activate K+ATP channels when added to the inside of the membrane (n=15). These actions are, in part concentrationwith the pmlominant action of low concentrations being associated with channel blockade. whilst higher concentrations with channel activation, but we also found that the effects were dependent upon the degree of channel integrity or 'run-down'. Indeed 5-HD was even able to reactivate desensitised channels following complete run-down.Similar complex effects were found in the presence of cytoplasmic nucleotides @=lo). In intact cells the effects of 5-HD were associated with a marked increase in [Ca2+]i (n=3) 7he results of this study indicate that 5-hydmxydecanoate has actions upon p e l 1 AT-sensitive K+ channels that have not been previously described in cardiac myocytes. Inhibition and activation by 5-HD may involve both direct interaction with the ion channels, and indirect effects possibly mediated by endogenous regulators of K + A~ channels such as protein kinases, protein phosphatases, etc. Since 5hydmxydecanoate will also elevate [Ca2+]i in these cells. it is also possible that the compound may have a significant influence on the regulation of insulin secretion.

Tolbutamide stimulates exocytosis of glucagon by inhibition of a mitochondrial‐like ATP‐sensitive K+ (KATP) conductance in rat pancreatic A‐cells

“…6). Consistent with results reported by others (Jagger et al 1993), these effects of 5‐HD on exocytosis were not paralleled by an inhibitory action of plasma membrane K ATP channel activity (not shown).…”

Intracellular Location of KATP Channels and Sulphonylurea Receptors in the Pancreatic β-cell: New Targets for Oral Antidiabetic Agents

“…Furthermore, the use of diazoxide and 5-HD presents additional restrictions for the characterization of these channels in intact β-cells. Whereas these compounds are used in cardiac cells as potent modulators of mitoK ATP with substantially less or no effect on sarcK ATP [66], in β-cells diazoxide is a potent inhibitor of plasma membrane K ATP channels, and 5-HD produces multiple effects [79]. Thus, it is necessary to address this issue with new strategies.…”

Multiplicity of effectors of the cardioprotective agent, diazoxide

“…Moreover, 5HD may have other off-target effects (e.g., cardiac mitochondria can directly metabolize 5HD (Hanley, et al, 2002; Suleiman, et al, 2001)). The selectivity of diazoxide to affect only mK ATP channels is increasingly being questioned (Jaggar, et al, 1993; Li, et al, 2010; Moritani, et al, 1994; Notsu, Ohhashi, et al, 1992; Notsu, Tanaka, et al, 1992; K. Sakamoto, et al, 1998; Szewczyk, et al, 2010).…”