SWIMMING PERFORMANCE, VENOUS OXYGEN-TENSION AND CARDIAC-PERFORMANCE OF CORONARY-LIGATED RAINBOW-TROUT, ONCORHYNCHUS-MYKISS, EXPOSED TO PROGRESSIVE HYPOXIA

We performed in vivo studies to examine the idea that cardiac work is impaired in rainbow trout (Oncorhynchus mykiss) below a certain venous Po-2 threshold. We hypothesized that coronary-ligated fish, swimming continuously at a reasonably high water velocity (1.5 body lengths.s(- 1)) and exposed to progressive hypoxia, would fatigue at higher venous Po-2 and ambient water Po-2 compared with sham-operated fish. However , we found that both the lowest venous Po-2 that supported hypoxic swi mming (9.9 torr for coronary-ligated fish and 11.1 torr for sham-opera ted fish) and the venous Po-2 at fatigue (7.8 torr and 8.6 torr, respe ctively) were the same for coronary ligated and sham-operated fish. Al so, both groups quit swimming at the same water Po-2, heart rare and h ematocrit. Nevertheless, significant differences in cardiac performanc e did exist between the two groups. Whereas ventral aortic blood press ure (P-va) increased significantly with hypoxic swimming in sham-opera ted fish, there was no such increase in coronary-ligated fish. In addi tion, cardiac arrhythmias occurred in coronary-ligated fish at fatigue , and these fish were slower to recover from exhaustion. We believe th at the venous Po-2 threshold to support cardiac performance in the abs ence of a coronary supply was between 7.8 and 9.9 torr. Furthermore, w e suspect that the low P-va in coronary-ligated fish effectively lower ed their myocardial O-2 demand. Uncertainty still exists regarding whe ther or not the venous Po-2 threshold lies between 8.6 and 11.1 torr i n sham-operated fish. (C) 1998 Elsevier Science Inc.