Renal hypertrophy in vivo is achieved by an increase in protein content per cell and an increase in cell size with minimal hyperplasia. Hypertrophied renal tubular cells remain quiescent and demonstrate an increase in transcellular transport rates. This situation was simulated in vitro by exposing a confluent, quiescent primary culture of rabbit renal proximal tubular cells to either insulin, prostaglandin E1, or hypertonic NaCl for 24 or 48 hr. Protein per cell increased by 20-30% with little or no increase in [3H~thymidine incorporation into DNA. Mean cell volume was also increased in insulin-and hypertonic NaCl-treated but not in prostaglandin Eltreated cells. The lag period required to initiate DNA synthesis by a combination of insulin and hydrocortisone was the same in control and hypertrophied cells, indicating a quiescent state of the latter. Two hours of exposure to the growth stimuli increased amiloride-sensitive Na+ uptake, Na-dependent H+ efflux, and ouabain-sensitive Rb+ uptake, indicating that stimulation of Na+/H' antiport (exchange) occurs as an early event in their action. Hypertrophied cells continued to demonstrate enhanced Na+/H+ antiport after the growth stimuli were removed for 3 hr, by which time their acute effects are reversed.Removal of part of the kidney mass results in hypertrophy of the remaining renal tissue. The ensuing increase in kidney size is largely due to hypertrophy-i.e., an increase in protein per cell or protein per DNA, rather than hyperplasia (i.e., an increase in cell number or DNA content) (1). Although attempts have been made to identify kidney-specific growth factors or renotropins (2), an alternative explanation for the hypertrophy is that one or more local factors could sensitize renal cells to a permissive balance of circulating growth factors and that the local stimulus determines the organ specificity of the response. Since fluid transport across the proximal tubule is increased within 24 hr of uninephrectomy (3) and remains elevated in the hypertrophied proximal tubule (4), and since the activity of the Na+/H+ exchanger ("antiporter") in isolated brush border membranes is increased after subtotal nephrectomy (5,6), in the present study we examined whether an increased rate of Na+/H+ antiport is an early event in cell hypertrophy. Although an increase in the activity of the Na+/H+ antiporter has been associated with mitogenesis (7,(17)(18)(19)(20)(21)(22)(23) medium] supplemented with bovine insulin (5 pug/ml), human transferrin (1 ,ug/ml), and hydrocortisone (50 nM). Confluent monolayers were exposed to fresh basic medium (i.e., 50:50 DME/F-12 media containing transferrin) for 48 hr, which resulted in arrest in Go/G1. Experiments were performed by adding specific growth stimuli to this basic medium for 24 and 48 hr. Cells exposed to the basic medium alone acted as controls.Three stimuli to cell hypertrophy (denoted "growth stimuli") were added to the basic medium and studied: (i) insulin at 10 ,ug/ml; (it) PGE1 at 1 ,4M, and (iii) NaCl at 20 mM. The dose...

A B S T R A C T The present studies examined whether vasopressin increases prostaglandin biosynthesis in isolated rabbit cortical collecting tubules (CCT) and whether endogenous prostaglandin biosynthesis plays a role in modulating the response of this nephron segment to vasopressin. Three groups of studies were performed. In the first group, CCT and proximal straight tubules (PST) were incubated with [3H]arachidonic acid, and metabolites were separated and identified using silica gel thin-layer chromatography. CCT were capable of producing all of the major prostaglandins (PG)(PGE2> thromboxane B2 [TxB2]> PGF2,, > PGI2). PST produced significantly lesser quantities of these lipids. In the second group, radiolabeled arachidonic acid was incorporated into the phospholipid pool of both CCT and PST, vasopressin was added to the incubation medium, and metabolites were separated and identified as above. Vasopressin stimulated the release of all of the major prostaglandins in CCT but had no effect on PST. PGE release into the incubation medium, as assessed by a radioreceptor assay, increased 108%, and a vasopressin analogue, 1-desamino-8-D-arginine vasopressin, had a quantitatively similar effect. In the third group, a submaximal dose of vasopressin was administered to isolated, perfused CCT studied in the presence and absence of indomethacin to assess whether endogenous prostaglandins play a role in modulating the antidiuretic response to vasopressin. cess, basal prostaglandin synthesis was 63% lower, and vasopressin-stimulated prostaglandin synthesis 76% lower, than the synthesis observed in rabbits on a normal diet. Cyclooxygenase inhibition exposed a significant hydroosmotic response to a submaximal dose of vasopressin in CCT from DOCA-or KCI-loaded animals. With arachidonic acid in the bath, the same dose of vasopressin failed to elicit a hydroosmotic response in CCT from rabbits on a normal diet even in the presence of a cyclooxygenase inhibitor. However, removal of exogenous arachidonic acid, with a consequently lower rate of prostaglandin synthesis, allowed the cyclooxygenase inhibitor to enhance the hydroosmotic response to vasopressin in these tubules.We conclude from these studies that the rabbit CCT has the capacity to synthesize all of the major prostaglandins and that the rate of synthesis of these lipids is enhanced by vasopessin. Prostaglandin synthesis by the CCT is postulated to modulate the antidiuretic action of vasopressin via a closed feedback loop. The effectiveness of this feedback regulation is dependent upon the mineralocorticoid status of the animal, which determines the level of basal and vasopressin-stimulated prostaglandin synthesis by the CCT.

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