Stimulation of Na+/H+ antiport is an early event in hypertrophy of renal proximal tubular cells.

Abstract

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...

Tubular Effects of Insulin

“…For example, glucocorticoids (14), thyroid hormones (15) and metabolic acidosis (16) increased Na/H exchange in brush border membrane vesicles isolated from rats. Insulin and prostaglandin E1 stimulated Na/H exchange in cultured proximal tubular cells (8) . Gesek and coworkers demon Takahashi et …”

“…However, insulin effect on Na/H exchange is controversial. Fine and colleagues demonstrated that insulin increases Na/H exchange in primary cultures of rabbit proximal tubular cells (8). In contrast, Mellas and co-workers reported that insulin had no stimulatory effect on Na/H exchange in canine proximal tubular cells (9).…”

Preincubation in acid medium increases Na/H antiporter activity in cultured renal proximal tubule cells.

“…After achieving confluence (8-12 days), cells were rendered quiescent by removal of insulin and hydrocortisone for 48 hr prior to and during preincubation (10).…”

“…Fetal bovine serum was added to 3% (vol/vol) to the culture medium for the first 3 days to allow better cell attachment. After achieving confluence (8-12 days), cells were rendered quiescent by removal of insulin and hydrocortisone for 48 hr prior to and during preincubation (10).…”

The effect of ouabain, insulin, and cyclic AMP on the acidification of luminal fluid in the proximal tubule of bullfrog kidney.

“…The recent advance in micropuncture techniques of ion-selective microelectrodes has made it possible to measure the intra-and extra-cellular ion activities, examining the movement of Na + and H + across the brush border membrane (BORON and BoULPAEP, 1983;SAsAKI et al, 1985). In the previous in vitro studies, it was shown that Na + /H + exchange was inhibited by cyclic AMP (cAMP) in the brush border vesicle from proximal tubular cells of rat kidney (KAHN et al, 1985), or stimulated by insulin in primary cultures of proximal tubular cells (FINE et al, 1985). Recently in our laboratory, we found a specific binding of insulin to the bullfrog kidney mass and homogenate (unpublished observation).…”