Many cardiac diseases can be associated with alterations in the function and quantity of G proteins. We examined the gene expressions and protein levels of Gi-1alpha, Gi-2alpha, Gi-3alpha and G(s alpha) in ventricular myocardial preparations from rats 4-6 weeks after induction of diabetes with streptozotocin in comparison with those from age-matched control rats. Diabetic rat myocardium exhibited reductions in the protein levels of Gi-2alpha and Gi-3alpha by 22+/-2% and 57+/-2%, respectively. In diabetes, 22% and 53% reductions in myocardial mRNA levels of Gi-2alpha and Gi-3alpha were observed. Although a faint protein signal of Gi-1alpha was detectable, no apparent expression of mRNA for Gi-1alpha was found in either control or diabetic myocardium. The reduced protein and mRNA levels of Gi-2alpha and Gi-3alpha were prevented by insulin therapy. No change was found in the protein and mRNA levels of G(s alpha) in diabetic myocardium. In conclusion, diabetes leads to a differential regulation of protein expressions of G(i alpha) isoforms and G(s alpha) in ventricular myocardium. The reduced expression of Gi-2alpha and Gi-3alpha proteins can be explained, at least in part, by the decreases in the transcriptional levels.