The ontogeny of Na+ uptake in larval rainbow trout reared in waters of different Na+ content

Soluble adenylyl cyclase is an acid-base sensor in rainbow trout red blood cells that regulates intracellular pH and haemoglobin-oxygen binding

AIM

To identify the physiological role of the acid-base sensing enzyme, soluble adenylyl cyclase (sAC), in red blood cells (RBC) of the model teleost fish, rainbow trout.

METHODS

We used: (i) super-resolution microscopy to determine the subcellular location of sAC protein; (ii) live-cell imaging of RBC intracellular pH (pHi) with specific sAC inhibition (KH7 or LRE1) to determine its role in cellular acid-base regulation; (iii) spectrophotometric measurements of haemoglobin-oxygen (Hb-O2) binding in steady-state conditions; and (iv) during simulated arterial-venous transit, to determine the role of sAC in systemic O2 transport.

RESULTS

Distinct pools of sAC protein were detected in the RBC cytoplasm, at the plasma membrane and within the nucleus. Inhibition of sAC decreased the setpoint for RBC pHi regulation by ~0.25 pH units compared to controls, and slowed the rates of RBC pHi recovery after an acid-base disturbance. RBC pHi recovery was entirely through the anion exchanger (AE) that was in part regulated by HCO3 --dependent sAC signaling. Inhibition of sAC decreased Hb-O2 affinity during a respiratory acidosis compared to controls and reduced the cooperativity of O2 binding. During in vitro simulations of arterial-venous transit, sAC inhibition decreased the amount of O2 that is unloaded by ~11%.

CONCLUSION

sAC represents a novel acid-base sensor in the RBCs of rainbow trout, where it participates in the modulation of RBC pHi and blood O2 transport though the regulation of AE activity. If substantiated in other species, these findings may have broad implications for our understanding of cardiovascular physiology in vertebrates.

The ontogeny of Na+ balance during rapid smoltification in pink salmon (Oncorhynchus gorbuscha)

Pink salmon hatch in fresh water, but their highly anadromous life history requires them to migrate into the ocean immediately after gravel-emergence, at a very small size. During their down-river migration these larvae undergo rapid smoltification that completely remodels their osmoregulatory physiology. At this time, the larvae reportedly have high whole-body Na⁺ contents and we hypothesised that the active accumulation of internal Na⁺ occurs in preparation for ocean entry. Using a comparative approach, the present study characterised the ontogeny of Na⁺ regulation in larvae of the anadromous pink salmon and the fresh-water rainbow trout. Our results indicate that larvae from both species actively accumulated Na⁺; however, whole-body Na⁺ content was higher in rainbow trout larvae compared to pink salmon. The time-course of this response was similar in the two species, with highest Na⁺-uptake rates ([Formula: see text]) shortly after yolk sac absorption, but the mechanism of Na⁺ accumulation differed between the species. Rainbow trout larvae greatly increased [Formula: see text] to overcompensate for a large simultaneous increase in Na⁺-efflux rate ([Formula: see text]), whereas pink salmon mounted a smaller increase in [Formula: see text] while maintaining tight control over [Formula: see text], which is supported by a significantly lower paracellular permeability. Our results indicate that the transient accumulation of internal Na⁺ is not a unique feature of the highly anadromous life history in pink salmon and may be a common ontogenetic pattern during larval development in salmonids; and perhaps it is associated with the development of the cardiovascular system during the larvae's transition to a more active lifestyle.