Previous studies have indicated that alterations in cardiac ATP-sensitive potassium channels (KATP) can occur with cardiac hypertrophy. The goal of this study was to determine the effect of cardiac hypertrophy in spontaneously hypertensive rats (SHR) on the response to the cardioprotective agents diltiazem and cromakalim. Isolated rat hearts from 14-wk-old SHR, normotensive heterozygote Wistar-Kyoto (WKY), and Sprague-Dawley (SD) strains were subjected to 25 min of global ischemia and 30 min of reperfusion in the presence of vehicle (3-30 microM cromakalim or 0.1-1.0 microM diltiazem). SHR had heart weight-to-body weight ratios 40-50% greater than age-matched SD or WKY. Both diltiazem and cromakalim increased reperfusion contractile function in a concentration-dependent manner in SD rats as previously reported. Cromakalim and diltiazem caused similar improvements in reperfusion function in WKY rats and SHR. Cumulative lactate dehydrogenase (LDH) release during reperfusion was similar for vehicle-treated SD, WKY, or SHR hearts. LDH release was significantly attenuated by cromakalim and dilitiazem at all concentrations tested in SD and WKY hearts, whereas LDH release was not attenuated in SHR hearts by any concentration of cromakalim or diltiazem tested. Morphological assessment of hearts by light microscopy indicated that the severity and distribution of myocardial lesions were not affected by cromakalim in SHR hearts, compared with vehicle-treated SHR, supporting the LDH data. These results suggest that in SHR hearts, cromakalim and dilitiazan may exert much of their cardioprotective effects on the population of myocytes that are not irreversibly damaged.