Effects of Hypoxic Acclimation, Muscle Strain and Contraction Frequency on Nitric Oxide-Mediated Myocardial Performance in Steelhead Trout (Oncorhynchus mykiss)
Whether hypoxic acclimation influences nitric oxide (NO)-mediated control of fish cardiac function is not known. Thus, we measured the function / performance of myocardial strips from normoxia and hypoxia-acclimated (40% air saturation; ~ 8 kPa O2) trout at several frequencies (20 - 80 contractions min-1) and two muscle strain amplitudes (8 and 14%) when exposed to increasing concentrations of the NO donor sodium nitroprusside (SNP) (10-9 to 10-4 M). Further, we examined the influence of: 1) nitric oxide synthase (NOS) produced NO (by blocking NOS with 10-4 M L-NMMA); and 2) soluble guanylyl cyclase mediated, NOS-independent, NO effects (i.e., after blockade with 10-4 M ODQ), on myocardial contractility. Hypoxic acclimation increased twitch duration by 8-10% and decreased mass-specific net power by ~35%. However, hypoxic acclimation only had minor impacts on the effects of SNP and the two blockers on myocardial function. The most surprising result of this study was the degree to which contraction frequency and strain amplitude influenced NO-mediated effects on myocardial power. For example, at 8% strain 10-4 SNP resulted in a decrease in net power of ~30% at 20 min-1 but an increase of ~20% at 80 min-1, and this effect was magnified at 14% strain. This study: suggests that hypoxic acclimation has only minor effects on NO-mediated myocardial contractility in salmonids; is the first to report the highly frequency- and strain-dependent nature of NO effects on myocardial contractility in fishes; and supports previous work showing that NO effects on the heart (myocardium) are finely tuned spatio-temporally.