Isotopic glucose turnover was measured during euglycemic glucose clamp studies in spontaneously hypertensive rats and normotensive Wistar-Kyoto control rats, under pentobarbital sodium anesthesia. At an insulin infusion rate of 4 mU.kg-1 x min-1, glucose disposal rate and hepatic glucose output were not significantly different in either group, at a plasma glucose of 6 mM. However, steady-state plasma insulin levels in spontaneously hypertensive rats were more than double those in Wistar-Kyoto rats (2.34 +/- 0.16 [mean +/- SE] vs. 6.78 +/- 0.58 nM, P < 0.00005, n = 7 in each group). Additional studies (n = 30 in spontaneously hypertensive rats and n = 32 in Wistar-Kyoto control rats) were conducted to match insulin levels in the two groups over a wide range, using infusion rates of 0.5-10 mU.kg-1 x min-1. When the responses of glucose disposal rate and hepatic glucose output to insulin were analyzed from the standpoint of plasma insulin levels, a highly significant difference was seen in the response to insulin between spontaneously hypertensive rats and Wistar-Kyoto rats, for both glucose disposal rate (P < 0.000005) and hepatic glucose output (P = 0.00007). The slope of the regression line for glucose disposal rate versus plasma insulin was lower in spontaneously hypertensive rats compared with Wistar-Kyoto rats (2.652 vs. 4.864, P < 0.001), indicating that insulin stimulation of glucose disposal rate was reduced by 50% in spontaneously hypertensive rats.(ABSTRACT TRUNCATED AT 250 WORDS)