Rh vs pH: the role of Rhesus glycoproteins in renal ammonia excretion during metabolic acidosis in a freshwater teleost fish

Abstract

Increased renal ammonia excretion in response to metabolic acidosis is thought to be a conserved response in vertebrates. We tested the hypothesis that Rhesus (Rh) glycoproteins in the kidney of the freshwater common carp, Cyprinus carpio, play a crucial role in regulating renal ammonia excretion during chronic metabolic acidosis. Exposure to water pH 4.0 (72 h) resulted in a classic metabolic acidosis with reduced plasma arterial pH and , no change in P CO2 and large changes in renal function. Urine [NH 4 + ] as well as [titratable acidity-HCO 3 -] rose significantly over the acid exposure, but the profound reduction (fivefold) in urine flow rates eliminated the expected elevations in renal ammonia excretion. Low urine flow rates may be a primary strategy to conserve ions, as urinary excretion rates of Na + , Cl -and Ca 2+ were significantly lower during the acid exposure relative to the control period. Interestingly, renal Rhcg1 mRNA and protein levels were elevated in acid-exposed relative to control groups, along with mRNA levels of several ion transporters, including the Na + /H + exchanger, H + -ATPase and NaImmunofluorescence microscopy showed a strong apical Rhcg1 signal in distal tubules. Taken together, these data show that renal Rh glycoproteins and associated ion transporters are responsive to metabolic acidosis, but conservation of ions through reduced urine flow rates takes primacy over renal acid-base regulation in the freshwater C. carpio. We propose that an 'acid/base-ion balance' compromise explains the variable renal responses to metabolic acidosis in freshwater teleosts.

Physiological and molecular responses of the goldfish kidney (Carassius auratus) to metabolic acidosis, and potential mechanisms of renal ammonia transport

“…In contrast to previous studies in which interpretation was confounded by a decrease in UFR (Wright et al, 2014) and/or an absence of ammonia filtration and secretion measurements (McDonald and Wood, 1981;Wood et al, 1999;King and Goldstein, 1983), the present study clearly revealed the pathways of increased urinary ammonia excretion. In confirmation of our first hypothesis, the elevation in urinary NH 4 + excretion was predominately mediated through increased renal tubular secretion (∼75%), though increased glomerular filtration of ammonia also made a significant contribution (∼25%).…”

“…In contrast to the common carp, which shut down UFR during acid exposure (Wright et al, 2014), goldfish tended to increase UFR, though by 36-48 h, the overall increase was not significant relative to simultaneous controls ( Fig. 1).…”

“…However, information on the mechanisms of renal ammonia transport and acid-base regulation in fish is sparse. To date, mRNA transcripts of the Rh family glycoprotein Rhbg and multiple isoforms of Rhcg have been found in the kidney of the common carp (Wright et al, 2014) and mangrove killifish , while Rhbg expression has been reported in the trout kidney . Immunohistochemical techniques have localized Rhcg-b (formerly termed Rhcg1 or Rhcg1b, see http://zfin.org/search?…”

“…These observations suggest the presence of a Na + -coupled mechanism of renal ammonia secretion similar to that of the gills ). However, the only study to examine this issue found an elevation in urine [ammonia] and increased expression of Rhcg-a (formerly Rhcg1a or Rhcg3) and Rhcg-b mRNA in the kidney of the common carp during metabolic acidosis, yet there was no relationship between urinary ammonia excretion and Na + excretion (Wright et al, 2014). But, interpretation was confounded by a simultaneous decrease in urine flow rate under acid exposure such that there was no increase in urinary ammonia excretion in this species.…”

“…Secondly, in light of the possible linkage of ammonia secretion via Rh proteins to Na + reabsorption discussed earlier, we hypothesized that increased expression of apical Rh glycoprotein (Rhcg-a and Rhcg-b) mRNA, decreased urinary Na + excretion, and increased renal tubular Na + reabsorption would occur. To test these hypotheses, low environmental pH (4.0) was employed as a tool for inducing a sustained metabolic acidosis; in this circumstance the gills take up rather than excrete acidic equivalents, and the kidney becomes the sole route of net acid excretion (McDonald and Wood, 1981;McDonald, 1983;King and Goldstein, 1983;Wood et al, 1999;Wright et al, 2014). Animals were fitted with urinary bladder catheters and exposed to control pH 8.2 or acid pH 4.0 water.…”