Modulation of intracellular calcium by potassium channel openers in vascular muscle.
We investigated two putative K+ channel openers, pinacidil and BRL34915 (cromakalim), and demonstrated their vasorelaxant effectiveness on rat artery contractions induced by K+, tetraethylammonium (TEA), or norepinephrine. The K+ channel opener-induced decrease in tension was rapid, even when tension was stimulated by 100 mmol/l K+. Measurements of intracellular free Ca++ (activity) by ultra-high sensitivity digital imaging microscopy was carried out by briefly loaded fura 2 (fluorescence ratio) quantitation in isolated, contracting cells of rat azygos vein. Submicron resolution was achieved by measuring cytoplasmic Ca(++)-sensitive fluorescence at each pixel, and size and intensity of areas with high Ca++ concentrations, called hot spots, were determined by a computer-generated, 3 lambda algorithm. Hot spots, which most likely represent the sites of Ca++ release and re-uptake by Ca(++)-regulatory organelles, increased in size and intensity upon addition of K+ or norepinephrine, reaching an early peak prior to the whole cell average peak in cytoplasmic Ca++ activity. Both norepinephrine and K(+)-induced stimulation resulted in Ca++ activity increases that were primarily due to Ca++ release from storage sites. Reduction of free Ca++ activity to resting or lower levels occurred upon addition of pinacidil or cromakalim. Intracellular Ca++ decreases due to K+ channel openers appeared abruptly beginning at the central portions of the cells, resulting in a pronounced early drop in central Ca++ activity while elevated Ca++ levels persisted at the periphery. While this late stage residual of peripheral Ca++ appears to be a significant step in the vascular muscle relaxant action of both K+ channel opener drugs, the level of Ca++ at peripheral sites was greater in response to pinacidil than to cromakalim. The results of this study suggest that in addition to increasing K+ conductance, pinacidil and cromakalim cause 1) decreased Ca++ activity in central regions of the myocytes, and 2) a shift in Ca++ distribution to primarily subsarcolemmal sites. These observations lead us to hypothesize separate control of peripheral and central Ca++ activity within a vascular muscle cell, with Ca++ redistribution that can be altered by vasorelaxants. We suggest that intracellular Ca++ redistribution may contribute the membrane potential-independent part of the vasorelaxant action of the K+ channel openers.
Erne
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P
Cardiovascular Research Laboratory, Earle A. Chiles Research Institute, Providence Medical Center, Portland, OR 97213.
Hermsmeyer
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K
D000818:Animals; D001578:Benzopyrans / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D002478:Cells, Cultured; D019806:Cromakalim; D006146:Guanidines / Q000494:pharmacology*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000378:metabolism; D009638:Norepinephrine / Q000037:antagonists & inhibitors / Q000494:pharmacology; D020110:Pinacidil; D015221:Potassium Channels / Q000187:drug effects*; D011758:Pyrroles / Q000494:pharmacology*; D051381:Rats; D011921:Rats, Inbred WKY; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000037:antagonists & inhibitors / Q000494:pharmacology; D014665:Vasodilator Agents / Q000494:pharmacology*
D001578:Benzopyrans; D006146:Guanidines; D015221:Potassium Channels; D011758:Pyrroles; D013757:Tetraethylammonium Compounds; D014665:Vasodilator Agents; D019806:Cromakalim; D019789:Tetraethylammonium; D020110:Pinacidil; D002118:Calcium; D009638:Norepinephrine