N a,K-ATPase activity and isoform expression were measured in rat small intestinal mucosa taken from both normal and streptozocin-treated diabetic rats. Enzyme activity and abundance was 1.7-2.3-fold higher in rats diabetic for 2 wk than in controls. This was associated with 1.4-1.7-fold increases in small intestinal protein and DNA content. Ouabain inhibition curves of Na,K-ATPase were monophasic with Kis of 2.6 +/- 1.4 x 10(-4) and 2.0 +/- 1.2 x 10(-4) M for control and diabetic rats, respectively (NS). Northern blot analysis revealed a 2.5-fold increase in mRNA alpha 1 and a 3.4-fold increase in mRNA beta 1 in diabetic rats relative to controls. Two thirds of this increase occurred within 24h after injection of streptozocin. Immunoblots of intestinal enzyme preparations from diabetic and control rats indicated the presence of alpha 1 and beta 1 subunits but not of alpha 2 or alpha 3. Administration of glucagon (80 micrograms/kg) to normal rats daily for 14-16 d increased mRNA alpha 1 3.1-fold but did not increase mRNA beta 1 or enzyme activity. In experimental diabetes, alpha 1 and beta 1 isoforms of Na,K-ATPase are coordinately upregulated at both protein and mRNA levels, an effect which appears to be partially mediated by the associated hyperglucagonemia.