Prolonged exposure of tissues to a receptor agonist often leads to adaptive changes that limit the subsequent responsiveness of the tissue to the same agonist. Recently, we have generated rats transgenic for the metallothionein I-human arginine vasopressin (AVP) fusion gene (Tg), which produced high plasma AVP with relatively preserved renal water excretion, suggesting that there might be adaptive mechanism(s) for maintaining water and electrolyte homeostasis against chronic AVP oversecretion from the earliest stage of life. In this study, to investigate whether down-regulation of AVP V2 receptor (V2R), which could possibly be caused by long-standing high plasma AVP, participates in this adaptive mechanism(s), non-peptidic V2R antagonist OPC31260 was administered to reverse the down-regulation, and water loading was performed after V2R antagonist treatment had been withdrawn. Additionally, to confirm the down-regulation, Northern blotting analysis for V2R mRNA was carried out. Tg rats showed slightly decreased urine volume and water intake with an equivalent plasma [Na(+)] level (Tg 140.4 +/- 0.6 mEq/l; control 139.3 +/- 0.6 mEq/l) under basal conditions. After water loading using a liquid diet containing zinc, which stimulates the promoter region in the transgene, the urine increase showed only limited suppression with a dramatically increased plasma AVP level and mild hyponatremia (135.8 +/- 1.8 mEq/l) in Tg rats. When diet containing OPC31260 had been provided for 4 days until the day before the start of water loading, antidiuresis and hyponatremia (125.4 +/- 1.mEq/l) were significantly potentiated. V2R mRNA expression in kidney was significantly less in Tg rats than in control rats under basal conditions, and this suppression was restored by OPC31260 treatment to levels comparable with those of control rats. These results suggest that long-standing high plasma AVP causes V2R down-regulation, and it may play an important role in the adaptive mechanism(s) for maintaining water and electrolyte homeostasis in chronically AVP-overexpressing rats.
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H Yokoi, H Nagasaki, K Tachikawa, H Arima, T Murase, Y Miura, M Hirabayashi, and Y Oiso
H Nagasaki, H Yokoi, H Arima, M Hirabayashi, S Ishizaki, K Tachikawa, T Murase, Y Miura, and Y Oiso
Arginine vasopressin (AVP) is a major antidiuretic hormone, the overproduction of which causes diluting hyponatremia in humans and is called the syndrome of inappropriate antidiuresis (SIAD). To study physiological changes resulting from AVP overproduction and to develop an animal model of hyponatremia, the human AVP gene was expressed under the control of the metallothionein promoter in transgenic (Tg) rats. Analyses of AVP immunoreactivity (irAVP) in the tissues revealed that the transgene is expressed mainly in the central nervous system. Gel filtration showed that irAVP in the brain and plasma was properly processed AVP. AVP purified from the brains of both Tg and control rats also exerted equal bioactivity to generate cAMP in LLC-PK1 cells. The founder rats did not show any physical or anatomical abnormalities. Under basal conditions, Tg rats had high plasma AVP levels (Tg 13.8 +/- 2.5 pg/ml; control 2.7 +/- 1.2 pg/ml; n=6 in both groups; means +/- S.E.M.), decreased urine volume, and normal plasma [Na(+)]. Hypertonic saline injected i.p. did not affect AVP secretion in Tg rats. In response to a zinc-supplemented liquid diet, plasma AVP decreased in control rats, but increased in Tg rats (Tg 32.7 +/- 2.7 pg/ml; control 1.0+/-0.1 pg/ml; n=6), resulting in hyponatremia (Tg 135.2 +/- 2.5 mEq/l; control 140.8 +/- 0.4 mEq/l; n=6). To our knowledge, this is the first transgenic animal to show diluting hyponatremia. This transgenic rat may therefore provide a useful model in which to investigate various physiological alterations resulting from the oversecretion of AVP which involve SIAD, stress response, behavior, and blood pressure.