TOLERANCE OF EPICARDIAL CORONARY ENDOTHELIUM AND SMOOTH-MUSCLE TO HYPERKALEMIA

Results of previous studies have suggested that high K+ concentrations in cardioplegic solutions may be detrimental to coronary endothelium in perfused hearts, as determined from changes in the coronary now rat e, but the direct functional changes in endothelium secondary to hyper kalemia have not been fully studied. To determine the effect of the K concentration in a physiologic solution (Krebs') and in St. Thomas' c ardioplegic solution, and the effect of exposure time on endothelium a nd smooth muscle, porcine coronary artery rings were set up in organ b aths under a physiologic pressure. The effect of exposure to Krebs' so lution containing 5.9 or 50 mmol/L K+ or to St. Thomas' solution conta ining 16 or 50 mmol/L K+, for either 2 hours (group I) or 4 hours (gro up II), was examined. The solutions were continuously aerated with 95% oxygen and 5% carbon dioxide to exclude the effects of ischemia and h ypoxia. The rings were then washed and contracted with K+ (25 mmol/L). The ability to release endothelium-derived relaxing factor (EDRF) in response to an EDRF stimulus (substance P) was used as an index of end othelial function. Smooth muscle function was evaluated in terms of th e K+-induced contraction force and the relaxation induced with glycery l trinitrate, in addition to the maximal substance P-induced relaxatio n. The maximal relaxation induced by substance P did not decrease by i ncubation with 50 mmol/L K+ in any group (p > 0.05). However, in group II rings incubated for 4 hours in Krebs' solution containing 50 mmol/ L K+, the contraction force to 25 mmol/L K+ (11.0 +/- 1.4 g) was signi ficantly less than that in the rings incubated with Krebs' solution (1 7.5 +/- 1.2 g; p < 0.05). There was no difference in the maximal relax ation induced by glyceryl trinitrate (p > 0.05). While the rings were incubated, St. Thomas' solution brought about slight relaxation of the rings, and no changes in either endothelium or smooth muscle function were seen even with an elevated K+ level of 50 mmol/L. These results suggest that (1) the endothelium and smooth muscle of the porcine coro nary artery are relatively resistant to hyperkalemia; (2) coronary end othelium may be more tolerant to hyperkalemia than is smooth muscle; a nd (3) coronary artery smooth muscle damage caused by hyperkalemia dur ing prolonged (4 hours) incubation may be prevented by cardioplegia.