The effects of hypercapnia were studied in cat papillary muscles driven at a frequency of 12 beats/min. The Pco2 of the medium was changed from 25 mm to 95 mmHg. Developed tension (DT) and maximal rate of tension development (dT/dt max) were recorded during a 30-minute period. Under isometric conditions and at 30 °C, increasing Pco2 resulted in a biphasic phenomenon Characterized by a decrease in DT and dT/dt max, followed by a partial recovery. Similar results were obtained under isotonic conditions, reserpine pretreatment or addition of propranolol. In the latter experiments, however, values of maximum depression and after 30 minutes of hypercapnia, were significantly lower than without the βblocking agent. At 22 °C and 22 °C plus reserpine, maximum contractility depression was greater and occurred later during the hypercapnic period than at 30 °C. In addition, at 22 °C plus reserpine, the degree and rate of recovery were significantly lower than at 30 °C. High calcium concentration diminished the initial depression of contractility and both high and low calcium decreased the degree and rate of contractility recovery. The present results suggest that : (1) trie initial negative inotropic effect of Pco2 seems to be dependent upon external calcium concentration, (2) this biphasic phenomenon cannot be attributed to transient changes in the duration of contraction nor in the stiffness of the series elasticity of the muscle, (3) the recovery phase can take place without catecholamines participation (intrinsic recovery) and (4) this intrinsic recovery process appears to be temperature- and calcium-dependent.
ASJC Scopus subject areas
- Physiology (medical)