The effect of X-537A (lasalocid) on potassium contracture in rat ventricular muscle

The effects of the carboxylic ionophore X-537A on contracture induced by high external K+ were determined in stimulated and unstimulated rat papillary muscles. X-537A (5 X 10(-6) M - 10(-5) M) increased peak K+-contracture force in stimulated but not unstimulated muscle. X-537A may alter myocardial force development by influencing transport of Ca2+ across the sarcolemma or Ca2+ release from sarcoplasmic reticulum.