Cellular pharmacology of potassium channel openers in vascular smooth muscle

Potassium channel opening is a physiological mechanism which excitable cells exploit to maintain or restore their resting state. Thus drugs that open vascular potassium channels have the potential to restrain or prevent contractile responses to excitatory stimuli or clamp the vessel in a relaxed condition. Hence, potassium channel openers, such as aprikalim and levcromakalim, relax agonist precontracted vascular preparations in vitro and lower systemic and regional vascular resistances in intact animals. Glibenclamide, a blocker of ATP sensitive potassium (KATP) channels, antagonises these effects. The main vasorelaxant mechanism of the potassium channel openers is to increase the potassium efflux through opening plasmalemmal potassium channels, which repolarises and/or hyperpolarises the membrane. This effect lowers the opening probability of voltage dependent calcium channels, restrains agonist induced calcium release from intracellular sources through inhibition of inositol trisphosphate formation, decreases the sensitivity of intracellular contractile elements to calcium, and accelerates the clearance of intracellular calcium via the Na+/Ca+ exchanger. Experimental evidence indicates that mechanisms not linked to potassium channel opening may also contribute to the potassium channel opener induced vasorelaxation; these remain to be clearly defined (for example, inhibition of the refilling of intracellular calcium stores). Potassium channel openers displace the binding of 3H-P1075, a potent potassium channel opener, in myocytes and intact rings from the rat aorta. In patches from vascular myocytes, potassium channel openers primarily open a small conductance (10-20 pS) KATP channel which is gated by [ATP]i and particularly by nucleotide diphosphates and magnesium.(ABSTRACT TRUNCATED AT 250 WORDS)