Activation of a relaxation cascade in isolated coronary arteries by brief electrical pulses.

Superfused cattle coronary artery rings contracted with endothelin or with a thromboxane A2 mimetic respond to transmural stimulation with relaxation. These relaxations are not reduced by denudation of the endothelium, or by combined pretreatment with quanethidine, atropine and propranolol, nor do they involve free radicals. Stimulation at 0.5 Hz with an increasing number of pulses, from 1 to 50, each of 500 microseconds in duration, produced progressively greater and more prolonged relaxations. Human coronary arteries, denuded and pretreated with guanethidine or tetrodotoxin, also showed prominent relaxations of both spontaneous and induced contractions with minimal transmural stimulation at 0.5 Hz. The delivery of 10 pulses briefly depressed near-maximal contractions to combinations of two or even three spasmogens in the cattle preparations. Reduction of extracellular potassium to near zero did not reduce stimulation-induced relaxations, but decreases in extracellular calcium did. The participation of ATP-sensitive potassium channels in stimulation-induced relaxations was unlikely because glibencamide did not reduce them, although it antagonized the relaxation produced by the potassium channel opener pinacidil. Tetraethylammonium blocked the relaxations to stimulation, indicating the involvement of calcium-dependent potassium channels, but block of small to moderate conductance SK channels with apamin was ineffective in antagonizing relaxations, making it likely that activation of large conductance SK channels or maxi-K channels was involved. Blockade of stimulation-induced relaxation in low sodium Krebs', but not by vanadate (1 mM), indicated the likely involvement of the Na+/Ca++ exchanger rather than a Ca(++)-ATPase as the calcium extrusion component of the relaxation process. Restoration of prestimulation tone required calcium influx through nifedipine-sensitive channels. It is concluded that a highly effective relaxation cascade is initiated by the delivery of one to five 0.5-msec pulses at 0.5 Hz, whose effects long outlast the stimulation parameters. The significance of this finding for coronary pathophysiology and as a means of selectively activating a relaxation cascade in an intact coronary artery segment is discussed.