Structure of the nephron and sites of action of hormones with special reference to hydro-mineral metabolism in the kidney of the bullfrog.

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The present study was projected to elucidate the sites of action for various hormones along the nephrons in the kidney of the bullfrog. Those hormones are arginine vasotocin (AVT), mesotocin (MST) and parathyroid hormone (PTH). On the basis of this study, morphological observation of the kidney was examined by light and electron microscopy and the reconstruction of the nephron was obtained from a serial section of the kidney of a bullfrog. Then, I used a microtechnique that allows hormone-dependent adenylate cyclase activity to be measured in samples containing a localized piece of tubules isolated by microdissection from collagenase-treated kidney tissue.1. MorphologyEach nephron consists of the following parts : renal corpuscle, ciliated neck segment, proximal tubule, ciliated intermediate segment, distal tubule, connecting tubule, and collecting duct. The cellular ultrastructures of the early distal and the connecting tubules have resemblance to those of the mammalian thick ascendin … More g limb and of the cortical collecting tubule, respectively.2. MethodologyCellular production of cAMP in response to hormone was examined in microdissected segments of the nephron.3. Effects of Hormones(1) AVT : The basal levels of cAMP were not significantly different among the segments (renal corpuscle, proximal, distal and connecting tubules). However, the basal levels were higher in spring than in late autumn. When AVT (1 muM) was added to the incubation medium, AVT activated the production of cAMP in both distal and connecting tubules but not in the renal corpuscle and proximal tubule. However, no appreciable responses to AVT were observed in the early distal tubules from frogs collected in late autumn. These results suggest that V_2-type receptors may be present in both distal and connecting tubules of the bullfrog kidney. Seasonal differences may exist with respect to the responsiveness to AVT of the nephrons of the bullfrog.(2) PTH : There was no stimulation of cAMP formation by PTH in both. the proximal and the distall tubules. The lack of hormonal stimulation of cAMP formation observed in both segments of tubules might reflect either the total absence of hormonal receptors associated either a cAMP-generating system in these preparations, their presence in only very small quantity, or the fact that they are not detectable by the methods used in this study.(3) MST and calcitonin : As study of these hormones is now being carried out, enough data to get conclusion are not available. Less