Abstract
Atrial natriuretic peptide (ANP) is known to influence NaCl transport in the medullary thick ascending limbs (MAL), where the largest NaCl reabsorption occurs among distal nephron segments in response to arginine vasopressin (AVP). In the present study, we investigated the effect of ANP on bicarbonate (HCO3 −) transport in the MAL using an isolated tubule perfusion technique. The HCO3 − concentration was measured using free-flow ultramicro-fluorometer. We first observed basal HCO3 − reabsorption in both long- and short-looped MALs (lMALs, and sMALs, respectively). AVP inhibited HCO3 − reabsorption in both lMALs and sMALs, whereas ANP did not change HCO3 − transport. However, in the presence of AVP, ANP restored the HCO3 − reabsorption inhibited by AVP both in lMAL and sMAL. The effects of ANP on HCO3 − transport was mimicked by cyclic GMP. The mRNA expression level of the vasopressin V2 receptor in lMALs was significantly higher than in sMALs, whereas expression of the V1a receptor was unchanged. In summary, AVP inhibits HCO3 − transport, and ANP counteracts the action of AVP on HCO3 − transport both in lMALs and sMALs.
Highlights
Arginine vasopressin (AVP) plays a central role in urine concentration and dilution by the kidney [1,2,3]
We have previously shown that AVP-stimulated NaCl reabsorption occurs only in lMALs not in sMALs [11]
Because there was no fluid transport in the lMAL and sMAL as we reported previously, the net bicarbonate transport (J TCO2) was calculated as J TCO2 = (CO-CL)VL/L, where CO and CL are the HCO32 concentration in the collected fluid and perfusate, respectively, VL is the rate at which fluid is collected at the end of the tubule, and L is the tubule length
Summary
Arginine vasopressin (AVP) plays a central role in urine concentration and dilution by the kidney [1,2,3]. AVP is known to stimulate NaCl reabsorption in the medullary thick ascending limbs (MAL) where AVP-stimulated Cl reabsorption is highest among the distal nephron segments [4,5,6,7,8]. The functional differences between lMALs and sMALs are not well known [10,11]. We have previously shown that AVP-stimulated NaCl reabsorption occurs only in lMALs not in sMALs [11]. It appears that lMALs have a more important role in urine concentration than do sMALs [10]
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