BETA-RECEPTORS AND STRESS PROTEIN-70 EXPRESSION IN HYPOXIC MYOCARDIUMOF RAINBOW-TROUT AND CHINOOK SALMON

We examined the in vivo effect of acute hypoxemia on myocardial cell-s urface (sarcolemmal) beta-adrenoreceptor density (B-max) and binding a ffinity (K-D) and on stress protein 70 (sp70) expression by exposing r ainbow trout (Oncorhynchus mykiss; 2.1-2.7 kg) to hypoxic water (3 mg/ l O-2) at 15 degrees C for 6 h. This degree of hypoxia was the minimum O-2 level that these trout could tolerate without losing equilibrium and struggling violently. Hypoxic exposure reduced arterial PO2 (Pa-O2 ) from 98 to 26 mmHg and arterial oxygen content (Ca-O2) from 10.8 to 7.4 vol/100 vol, but did not elevate epinephrine and norepinephrine le vels above 10 and 90 nM, respectively. Despite the substantial reducti on in blood oxygen status, the B-max and K-D of myocardial cell-surfac e beta-adrenoreceptors were unaffected by 6 h of hypoxic exposure. In addition, acute hypoxemia did not increase myocardial sp70 expression. The failure of short-term hypoxia to decrease trout myocardial beta-a drenoreceptor density clearly contrasts with the established hypoxia-m ediated down-regulation shown for mammals. To further investigate the influence of low PO2 on salmonid myocardial beta-adrenoreceptors, bind ing studies were performed on the spongy (continuously exposed to deox ygenated venous blood) and compact (perfused by oxygenated blood suppl ied by the coronary artery) myocardia of chinook salmon. The spongy my ocardium has adapted to its microenvironment of continuous low PO2 by having 14% more cell-surface beta-adrenoreceptors compared with the co mpact myocardium. There was no tissue-specific difference in XD and no evidence of sexual dimorphism in B-max or K-D. We conclude from our s tudies that the salmonid heart is well adapted for sustained performan ce under hypoxic conditions. We found that wild chinook salmon had 2.8 x more cell-surface beta-adrenoreceptors compared with hatchery-reared rainbow trout. This difference suggests a significant degree of plast icity exists for fish myocardial beta-adrenoreceptors. The signals und erlying such differences await further study, but are not likely to in clude moderate hypoxia and sexual dimorphism.