Insulin Binding and Degradation by Luminal and Basolateral Tubular Membranes from Rabbit Kidney

Abstract

A B S T R A C T Insulin influences certain metabolic and transport renal functions and is avidly degraded by the kidney, but the relative contribution of the luminal and basolateral tubular membranes to these events remains controversial. We studied 125I-insulin degradation [TCA and immunoprecipitation (IP) methods] and the specific binding of the hormone by purified luminal (L) and basolateral (BL) tubular membranes. These were prepared from rabbit kidney cortical homogenates by differential and gradient centrifugation and ionic precipitation steps in sequence, which resulted in enrichment vs. homogenate of marker enzymes' activities (sodium-potassium-activated adenosine triphosphatase for BL and maltase for L) of 8-and 12-fold, respectively. Both fractions degraded insulin avidly and bound the hormone specifically without saturation even at pharmacologic concentrations (10 lM). At physiologic insulin concentrations (0.157 nM) BL membranes degraded substantial amounts of insulin (44.2±2.6 and 40.7±2.2 pg/mg protein per min by the TCA and IP methods, respectively), even though at lesser rates (P <0.001) than the luminal fraction (67.2±2.3 and 75±6.2 pg/mg protein per min, respectively); the rate of insulin catabolism by BL membranes was significantly higher (P < 0.001) than that which could be attributed to their contamination by luminal components [12.2±1.9 pg/mg per min (TCA method), or 13.7±1.9 pg/mg per min (IP method)]. Competition experiments suggested that insulin-degrading activity in both fractions includes both specific and nonspecific components. In contrast to degradation, insulin binding by both membranes was highly specific for native insulin and was severalfold higher in BL than L membranes [17.5±1.3 vs. 4.5±0.4 fmol/mg protein (P < 0.001) at physiologic insulin concentrations]. Despite the marked difference in the binding capacity for insulin by the two membranes,

Insulin stimulates volume absorption in the rabbit proximal convoluted tubule.

“…Renal cortical membranes have high affinity receptors that bind insulin at physiologic concentrations (6-1 1). Insulin receptors are present on both the apical and basolateral membranes, but binding is much greater in basolateral membranes (8). Insulin binding not only affects solute transport as demonstrated in this study, but also is important for insulin degradation (7)(8)(9).…”

“…Insulin binding not only affects solute transport as demonstrated in this study, but also is important for insulin degradation (7)(8)(9). Although binding is more avid on the basolateral membrane, insulin degradation occurs at a greater rate on the apical membrane (8).…”

“…Briefly, kidneys from female New Zealand white rabbits were cut in coronal slices. Individual tubules were dissected in cooled (40C) ultrafiltrate-like solution containing 115 mM NaCl, 25 mM NaHCO3, 2.3 mM Na2HPO4, 10 mM Na acetate, 1.8 mM CaCl2, 1 mM MgSO4, 5 mM KCl, 8.3 mM glucose, and 5 mM alanine.…”

Metabolic syndrome and renal sodium handling in three ethnic groups living in England

“…In humans, insulin has an acute antinatriuretic effect [20], even in obese persons resistant to other metabolic effects of the hormone [21]. Although insulin is thought primarily to affect distal tubular sodium re-absorption [21,22,23,24], there is evidence of a direct insulin action on proximal tubular sodium re-absorption [25,26]; moreover, insulin receptors have been found in the proximal tubule of different species [27,28,29]. Finally, insulin increases the expression and the activity of sodium-hydrogen exchange isoform 3, which mediates more than 60% of the sodium re-absorption at the proximal tubule [30].…”

Tubular Effects of Insulin

“…The apical membrane as well as the basolateral membranes of the proximal tubules have insulin reeptors (18,19). We also tested the effect of insulin n the lumen and obtained the result that pHi was nchanged (data not shown).…”