Effect of pH on trout blood vessels and gill vascular resistance

pH is recognized as a modulator of vascular smooth muscle (VSM) tone in mammalian vessels, but little is known about its effects on fish VSM. We investigated the effects of extracellular and intracellular pH (pH(o) and pH(i), respectively) on isolated vessels from steelhead and rainbow trout (Oncorhynchus mykiss, Skamania and Kamloops strains, respectively) and of pH(o) on perfused gills from rainbow trout. In otherwise unstimulated (resting) efferent branchial (EBA) and coeliaco-mesenteric arteries (CMA), anterior cardinal veins (ACV) and perfused gills, increasing pH(o) from 6.8 to 8.8-9.0 produced a dose-dependent contraction or increase in gill resistance (R(GILL)) with an estimated half-maximal response of 8.0-8.2. pH(o) interactions with other contractile stimuli were agonist specific; more force was developed at low pH(o) in ligand-mediated (arginine vasotocin) contractions, whereas depolarization-mediated (40-80 mmol l(-1) KCl) contractions were greatest at high pH(o). Increasing pH(i) by application of 40 mmol l(-1) NH4Cl produced sustained contraction in afferent branchial arteries (ABA) suggesting that these vessels could not readily restore pH(i). NH(4)Cl application only transiently contracted EBA and CMA in Hepes buffer, whereas it produced a slight, but prolonged, relaxation of EBA and CMA in Cortland buffer. The buffer effect was due to the presence of Hepes and in this environment EBA and CMA appeared to readily restore pH(i). Increasing pH(i) in KCl-contracted EBA in Hepes produced an additional contraction, whereas ligand-contracted (thromboxane A2 analog, U-46619) EBA relaxed. Reducing pH(i) (NH(4)Cl washout) transiently contracted resting EBA and CMA in both Hepes and Cortland buffer. NH4Cl washout produced an additional, transient contraction of both KCl- and U-46619-contracted EBA in Hepes. EBA contractions produced by increased pH(i) depend primarily on intracellular Ca2+, whereas both intracellular and extracellular Ca2+ contributed to the response to decreased pH(i). Three cycles of perfusate acidification (pH(o) 7.8 to 6.2 and back to 7.8) reproducibly halved, then restored R(GILL) with no adverse effects, indicating that this was not a pathophysiological response. These studies show that the general effects of pH on VSM are phylogenetically conserved from fish to mammals but even within a species there are vessel-specific differences. Furthermore, as fish are exposed to substantial fluctuations in environmental (and therefore plasma) pH, the obligatory response of fish VSM to these changes may have substantial impact on cardiovascular homeostasis.