Na+/Ca2+ exchange in plasma membrane vesicles from a glucose-responsive insulinoma

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

No evidence for a role of reverse Na(+)-Ca2+ exchange in insulin release from mouse pancreatic islets

We studied whether reverse Na(+)-Ca2+ exchange can increase cytoplasmic Ca2+ ([Ca2+]i) in mouse islets and contribute to insulin release. The exchange was stimulated by replacing Na+ with choline, sucrose, or lithium in a medium containing 15 mM glucose. Na+ omission increased electrical activity in B cells, [Ca2+]i, and insulin release. When voltage-dependent Ca2+ channels were blocked by nimodipine or closed by holding the membrane polarized with diazoxide, Na+ omission caused a slight hyperpolarization, a small rise in [Ca2+]i, and a marginal increase in insulin release (the latter only with choline). This small rise in [Ca2+]i was dependent on extracellular Ca2+ but was hardly augmented when intracellular Na+ was raised with alanine. When B cells were depolarized by 30 mM K+, Na+ omission did not affect the membrane potential but increased [Ca2+]i and insulin release. If Ca2+ channels were blocked by nimodipine, only marginal increases in Ca2+ and insulin release persisted, which were not different from those observed when the cells were not depolarized. This indicates that Ca2+ influx through voltage-dependent Ca2+ channels rather than via reverse Na(+)-Ca2+ exchange underlies the rise in [Ca2+]i and in insulin release produced by Na+ removal. No decisive support for Ca2+ influx by reverse Na(+)-Ca2+ exchange could be found.