Phosphocreatine and creatine kinase in energetic metabolism of the porcine carotid artery.

Magnetization exchange experiments and force analysis were performed on porcine carotid arteries with varied phosphocreatine (PCr) levels. The aim of these experiments was to determine the creatine kinase (CK) kinetics and the role in hypoxic relaxation. The magnetization exchange techniques used were multisite saturation transfer (MST) and conventional saturation transfer (CST). The two techniques were used because CST assumes a two-site exchange while MST allows one to assume a three-site exchange. Mechanical parameters of tension generation and relaxation were measured to determine the energetic effects on contractility of carotid strips. Measurements of molecular exchange between ATP and PCr found the pseudo first-order rate constant (kf) of 0.17 +/- 0.04 S-1 (PCr-->ATP) and kr = 0.12 +/- 0.03 S-1 (ATP-->PCr) in unstimulated porcine carotid artery. In the carotids, despite increased PCr and K+ stimulation, no magnetization exchange is observable with MST. This result indicates that the ATPase was less than 0.04 mumol/g/s (below the NMR resolution) while CK was 0.11 mumol/g/s. Creatine-loaded carotids showed no significant differences in force measurements: maximal force, resting tension, and the rate of hypoxia were all unchanged. The flux ratio (flux forward over flux reverse) was 0.94 +/- 0.13 which was considered to be indicative of CK being at equilibrium in the resting porcine carotid artery. The rate of the CK reaction is rapid enough to assume a two-site kinetic exchange not limiting energetic supply during hypoxia-induced relaxation.