Effects of PTH, calcitonin, and cAMP on calcium transport in rabbit distal nephron segments

Abstract

Distal nephron segments are heterogenous with respect to adenylate cyclase responses to stimulation with parathyroid hormone (PTH) or calcitonin (CT). We examined effects of these hormones and of 8-(p-chlorophenylthio)-adenosine 3',5'-cyclic monophosphate (CPTcAMP) on net Ca absorption (Jnet Ca2+, pmol.min-1.mm-1) in rabbit distal nephron segments by in vitro microperfusion technique. We studied three segments, including distal convoluted tubule (DCT), connecting tubule (CNT), and cortical collecting duct (CCD). PTH (1 nM) in bath significantly increased Jnet Ca2+ from 2.28 +/- 0.35 to 9.44 +/- 1.13 in CNT, but did not affect Jnet Ca2+ in DCT or CCD. CT (1 nM) in bath significantly increased Jnet Ca2+ from 1.58 +/- 0.29 to 4.45 +/- 1.01 in DCT, whereas it did not affect Jnet Ca2+ either in CNT or in CCD. CPTcAMP (30 microM) in bath significantly increased Jnet Ca2+ from 2.29 +/- 0.42 to 3.97 +/- 0.43 in DCT and from 2.43 +/- 0.18 to 5.83 +/- 0.37 in CNT, but it did not affect Jnet Ca2+ in CCD. When Na+ was removed from bathing fluid or when 0.1 mM ouabain was added to bath, Jnet Ca2+ in both DCT and CNT significantly decreased. Furthermore, stimulatory effects of PTH and CT on Ca2+ absorption in the respective segments were abolished under these conditions. These results suggest that PTH and CT increase Ca2+ absorption in CNT and DCT, respectively, through cAMP-mediated mechanisms. Presence of a basolateral Na(+)-Ca2+ exchange process seems to be a prerequisite for effects of these hormones. However, exact intracellular mechanisms remain uncertain.

Co-ordinated control of apical calcium influx and basolateral calcium efflux in rabbit cortical collecting system

“…The second m essenger cAMP has been frequently implicated as th e m ediator of horm one-stim ulated Ca2+ reabsorption in the distal tubule [5,24,25]. As demon strated in the present study, activation of adenylyl cyclase by forskolin indeed rapidly enhanced apical Ca2+ influx, increased [Ca2+], and stim ulated apical-tobasolateral 45Ca2+ flux.…”

“…The exchanger is capable of operating in a Ca2+ efflux (forward) and influx (reversed) m ode [17]. If th e rate of these basolateral Ca2+ pum ps is regulated by endogenous m odulators such as PTH, vasopressin and l, 25[OH]2D3, th en m aintenance of a tight coupling betw een apical influx an d basolateral efflux w ould require a cellular feedback signal to control Ca2+ influx a t the apical m em brane. [Ca2+]j could serve as this signal, analogous to its role in th e regulation of solute transport in various epithelia [18,19].…”

“…Knowledge of the local nature of these events is, therefore, of importance and is now becoming feasible through the use of high spatial resolution confocal laser scanning microscopy [19]. Thirdly, cytosolic calbindin-D2gk levels could also control the rate of transcellular Ca2+ transport [2], In a previous study, we have implicated calbindin-D28k in the long-term control of Ca2+ reabsorption by l, 25[OH]2D3 [23], It is conceivable th at the level of calbindin-D28k determines the maximal achievable rate of transcellular Ca2+ transport, but it is rather unlikely that in short-term regulation, as investigated in th e present study, calbindinDjak represents the rate-limiting step.…”

“…In this way, Ca2+ entry controls the cytosolic Ca2+ activity ([Ca2+]i), th e availability of Ca2+ to Ca2+ efflux systems, and the rate of transcellular Ca2+ transport. It is also feasible th at basolateral Ca2+ pumps are controlled directly by horm ones such as l, 25[OH]2D3 or PTH. In this latter idea, a cytosolic signal is needed to direct apical Ca2+ entry since the two opposing membranes must operate in concert to preserve [Ca2+], within reasonable limits during substantial rates of transcellular Ca2+ transport [3],…”

“…To this end, rabbit connecting tubule and cortical collecting duct cells were cultured to confluency on permeable supports. In previous studies, this primary culture has been shown to retain several characteristics of the original epithelium, including hormone-responsive transcellular Ca2+ reabsorption and l, 25[OH]2D3-dependent calbindin-D28k levels [5-cultured cells provide, therefore, a suitable system for detailed analysis of the process of transcellular Ca2+ transport.…”