Insulin Unresponsiveness of Tubular Monovalent Cation Transport during Fructose-Induced Hypertension in Rats

1. Hyperinsulinaemia is considered to be a pathogenic factor for human and experimental hypertension. Thus, the contribution of the known insulin-stimulated tubular sodium reabsorption to this aetiological process has to be discussed.

2. Rats fed a fructose-enriched diet develop hyperinsulinaemia and hypertension, providing a model for studying the regulation of the tubular sodium handling and its possible relationship to hypertension. For this purpose, the sodium transport capacity of isolated nephron segments from control rats and from rats fed a fructose-enriched diet was investigated by measurement of ouabain-sensitive ⁸⁶Rb uptake and of the hydrolytic activity of Na,K-ATPase. The number and affinity of insulin receptors were estimated from the specific [¹²⁵I]insulin binding.

3. In rats fed a fructose-enriched diet, mild hypertension developed during the 14-day fructose diet. There were no differences, along the nephron, in basal ⁸⁶Rb uptakes and ATPase activities between control rats and fructose-induced hypertensive rats. In control rats, insulin stimulated ⁸⁶Rb uptake in the proximal convoluted tubule and cortical collecting duct, but exhibited an inhibitory action in the medullary thick ascending limb. In contrast, in fructose-induced hypertensive rats, ⁸⁶Rb influx remained unresponsive to insulin concentrations ranging from 10⁻¹¹ to 10⁻⁷ mol/l in the proximal convoluted tubule and cortical collecting duct. In the medullary thick ascending limb, the threshold of inhibition was displaced from 10⁻¹¹ mol/l up to 10⁻⁷ mol/l. Insulin binding to the proximal convoluted tubule, medullary thick ascending limb and collecting duct were similar in control rats and in rats fed a fructose-enriched diet.

4. We conclude that hypertension developed in rats fed a fructose-enriched diet regardless of change in renal sodium handling since (1) the basal tubular sodium reabsorption capacity of the nephron remained unchanged and (2) the response of the tubular cation transport to insulin was abolished. These results strongly argue against the participation of insulin-mediated tubular sodium retention in the pathogenesis of hypertension and suggest that insulin-related mechanisms modulate the blood vessel reactivity.