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...