Effects of potassium on isolated canine coronary arteries. Modulation of adrenergic responsiveness and release of norepinephrine.

+ ),,, arteries exhibited calcium- induced relaxations and verapamil-induced contractions; (3) at |K* |,, between 5 and 15 mM there was a steep rise in force which was increased by isoproterenol (/3-adrenergic contraction), almost completely blocked by L-propranolol, but not attenuated by a-adrenergic blockade with phentolamine; (4) at |K' )« > 30 mM, effects of /3-adrenergic stimulation and blockade became very small. Steady increases in force elicited by isoproterenol and by sudden increases in (K + )o were preceded by transient relaxations. Effects of exogenous and endogenous (tyramine, nerve stimulation) norepinephrine paralleled those of isoproterenol and were blocked by propranolol but not attenuated by phentolamine. In contrast, modulation of the effects of phenylephrine by potassium consisted of monotonicaUy increasing constrictor responses over the whole range of |K'|0 tested. Arteries labeled with ( 3