Effect of Age and Methacholine on the Rate and Coronary Flow of Isolated Hearts of Diabetic Rats

1. Isolated hearts perfused by the method of Langendorff from 6, 12 and 24 week streptozotocin (STZ) diabetic rats displayed a significant bradycardia following 60 min equilibration. The rate of hearts from 12-week diabetic rats (164 +/- 17) displayed the greatest bradycardia compared to age-matched controls (268 +/- 15; P less than 0.001), and diabetics treated with insulin (232 +/- 17; P less than 0.01), but by 52 weeks the heart rate of the 3 groups was similar. With advancing age the effect of STZ diabetes on the rate of rat isolated perfused hearts remained unchanged but the rate of the control and diabetic + insulin groups declined. 2. Hearts from 6-52 week STZ-treated rats were found to be more sensitive to the negative chronotropic effect of methacholine, the greatest difference occurring in hearts from the 12 week animals. Atropine (10(-7) M) did not affect the resting heart rate of age-matched controls or diabetics but blocked methacholine (2.6 x 10(-6) M)-induced bradycardia in both, suggesting that the site of action of diabetic bradycardia is not the muscarinic receptors. 3. At the end of equilibration there was a significant decrease in coronary flow in hearts from 12 week diabetic animals. In spontaneously beating diabetic rat hearts administration of methacholine (2.6 x 10(-6) M) produced a significantly greater decrease in coronary flow in the 12, 24 and 52 week diabetic hearts. When electrically paced (5 Hz) however, there was no difference in response to methacholine between the three groups except at 52 weeks between the age-matched control and diabetic groups. This suggests that the more pronounced reduction induced by methacholine on the coronary flow of diabetic hearts is secondary to its negative chronotropic effect. 4. In general, hearts from diabetic animals treated with insulin respond similarly to their agematched controls in the presence and absence of methacholine.