African lungfish, Protopterus annectens, possess an arginine vasotocin receptor homologous to the tetrapod V2-type receptor

In tetrapods, arginine vasopressin and its counterpart, arginine vasotocin (AVT), are involved in renal water conservation through vascular V1a-type and tubular V2-type receptors, and only the former has thus far been cloned in fish. We successfully cloned the V1a-type and V2-type AVT receptor from the kidney of the African lungfish, Protopterus annectens, and the deduced amino acid sequences exhibited high homology with amphibian Via- and V2-type receptors, respectively. Functional analysis showed that AVT addition to CHO cells transfected with lungfish V1a-type receptor increased [Ca²⁺]j in a concentration-dependent manner, whereas CHO cells transfected with lungfish V2-type receptor responded with cAMP accumulation after AVT stimulation. Lungfish V2-type receptor mRNA was strongly expressed in the heart and kidney, while V1a-type receptor mRNA was ubiquitously expressed in all the tissues examined. In the kidney, immunohistochemistry using a specific antibody to lungfish V2-type receptor showed localization in the basolateral area of the cells in the late part of the distal tubules. Artificial estivation (EST) for 90 days significantly increased plasma osmolality and sodium and urea concentrations. There was no significant difference in the V2-type receptor mRNA and protein expression levels in the kidney between the freshwater and EST lungfish, while the AVT precursor mRNA level in the hypothalamus was remarkably higher in the EST lungfish. Our results indicate that African lungfish possess a functional V2-type receptor similar to that in tetrapods, suggesting that elevated plasma AVT during estivation exerts a renal tubular antidiuretic effect through the V2-type receptor expressed in the distal segments of lungfish kidney.