Myocardial adaptation to chronic propranolol therapy in diabetic rats

The myocardial adaptation to chronic beta-adrenergic blockade was explored in diabetic and control female Wistar rats. Propranolol was administered by intraperitoneal injection, 30 mg/kg every 12 h. Treatment began 2 months after streptozotocin injection and at a comparable time in controls. Diabetes reduced the heart rate by 90 beats/min; propranolol decreased the heart rate by 30 beats/min further in diabetic rats. Propranolol reduced the heart rate by 100 beats/min in controls. Study of isolated ventricular papillary muscle showed qualitatively similar effects of propranolol in control and diabetic animals. Diabetes resulted in prolonged contraction duration and decreased shortening velocity; both developed tension and peak shortening were unchanged. In contrast, propranolol resulted in prolonged contraction duration but no change in shortening velocity; both developed tension and peak shortening were increased. The shortening of isometric relaxation in response to norepinephrine was exaggerated with propranolol therapy in control but not diabetic rats. These findings indicate that the myocardial adaptation to chronic beta-blockade results in increased developed tension and increased extent of muscle shortening in vitro, and differs qualitatively from the adaptation to diabetes.