Mechanism of action of K channel openers on skeletal muscle KATP channels. Interactions with nucleotides and protons.

Abstract

The molecular mechanisms underlying the actions of K channel openers (KCOs) on KATe channels were studied with the patch clamp technique in excised inside-out patches from frog skeletal muscle fibers. Benzopyran KCOs (levcromakalim and SR 47063) opened channels partially blocked by ATP, ADP, or ATP~s, with and without Mg 2+, but they had no effects in the absence of internal nucleotides, even after channel activity had significantly declined because of rundown. The effects of KCOs could therefore be attributed solely to a competitive interaction between KCOs and nucleotides, as confirmed by observations that ATP decreased the apparent affinity for KCOs and that, conversely, KCOs decreased ATP or ADP sensitivity. Protons antagonized the action of the non-benzopyran KCOs, pinacidil and aprikalim, by enhancing their dissociation rate. This effect resembled the effect of acidification on benzopyran KCOs (Forestier, C., Y. Depresle, and M. Vivaudou. FEBS Lett. 325:276-280, 1993), suggesting that, in spite of their structural diversity, KCOs could act through the same binding sites. Detailed analysis of the inhibitory effects of protons on channel activity induced by levcromakalim or SR 47063 revealed that, in the presence of 100 p~M ATP, this effect developed steeply between pH 7 and 6 and was half maximal at pH 6.6. These results are in quantitative agreement with an allosteric model of the KAxp channel possessing four protonation sites, two nucleotidic sites accessible preferentially to Mg2+-free nucleotides, and one benzopyran KCO site. The structural implications of this model are discussed.

Vasorelaxant effects of the potassium channel opener SR 47063 on the isolated human saphenous vein and rat aorta

“…However, this relaxant effect was not observed in the presence of a high concentration of potassium (60 mM). Thus, SR 47063 exhibits a profile of pharmacological activity characteristic of K + -channel openers in a variety of vascular tissues including rabbit aorta, dog coronary and middle cerebral artery (13)(14)(15), and the current results are in agreement with previous studies with this drug on rat vascular (3,4), human bronchial (5) and frog skeletal muscle preparations (16,17).…”

Zinc is both an intracellular and extracellular regulator of KATP channel function

“…The potentiation of K ATP channels by extracellular Zn 2+ appeared specific to the pancreatic channel, since Zn 2+ failed to activate the cardiac form of K ATP channels (SUR2A/Kir6.2), and even elicited a weak inhibition of these channels at the highest concentration tested of 20 μ m. Kwok & Kass (1993) showed that such a concentration also inhibited native channels but to a much greater extent. Since they recorded Zn 2+ effects on pinacidil‐activated currents, the discrepancy could arise from a possible antagonism between K + channel openers and Zn 2+ , not unlike that between openers and protons (Forestier et al 1996).…”

ATP sensitive potassium channel openers: A new class of ocular hypotensive agents

“…(Light et al, 1994) These channels are composed of SUR2A and K ir 6.2 (Allard and Lazdunski, 1993; Forestier et al, 1996) and their degree of inhibition by ATP is strongly regulated by changes in pH. (Davies et al, 1992) Similar to K ATP channels present in other tissues, activation of these channels in skeletal muscles protects cells from excess Ca 2+ accumulation, subsequently regulating contractile properties.…”