Vascular Tone and Contractility During Exposure to Cardioplegia and Hyperkalemic Solutions

Abstract

Hyperkalemic cardioplegic solutions have been widely used for myocardial protection. A commonly accepted concept is that cardioplegic solutions may evoke vasospasm due to the high potassium (K+) concentration in the solutions. However, little has been known about coronary vascular tone and contractility during ischemia and reperfusion. The present study was designed to test the hypothesis that hyperkalemic cardioplegic solutions may increase the vascular tone during exposure and increase the contractility during reperfusion.

Porcine coronary artery (PCA) and neonatal rabbit aortic (RAO) rings were set up in organ baths under a physiologic pressure. In addition, the effect of reexposure to K+ on contractility was studied in the human internal mammary artery (HIMA). The solutions were continuously aerated with 95% 02 and 5% Co₂ to exclude the effects of ischemia and hypoxia. The effect of Krebs containing 5.9 or 50 mM K+, or St. Thomas' cardioplegic solution No. 2 (ST) containing 16 or 50 mM K+ on vascular tone for four hours was examined. In other groups, after two-hour-incubation, the rings (n= 5 in PCA and n= 6 in RAO in each subgroup) were washed and cumulative concentrationcontraction curves were established.

During four-hour-exposure to ST, the rings were relaxed (−0.38 ±0.07 g in PCA and -0.43 ±0.13 g in RAO, n = 6, P ≥ 0.001). In comparison with 50 mM K+ in Krebs, ST with 50 mM K+ induced a significantly reduced contraction with a slow onset in both vessels. After two-hour-exposure to ST the RAOs had almost unchanged contractility whereas those exposed to 50 mM K+ had enhanced contractility. Reexposure to K+ (100 mM) significantly enhanced the contraction in the HIMA.

These results suggest that despite its high K+ concentration St. Thomas cardioplegic solution does not evoke vasoconstriction during exposure in either PCA or RAO. In contrast, it relaxes these vessels during four-hour exposure. The study also demonstrates that although two-hour-exposure to extra−-K±added (50 mM) physiologic or cardioplegic solutions enhances the contractility in RAO, the exposure to ST is unlikely to do so. It remains to study whether microvasculature has similar reaction during and after exposure to hyperkalemia and whether combined factors such as ischemia affect the reaction.