[Effect of caffeine and theophylline on Ca2+ sensitivity of troponin and Ca2+ release from SR in saponin-skinned rat cardiac muscle].

Cell membrane of cardiac muscle fiber (rat right ventricular papillary muscle) was chemically skinned (permeability of membrane increased) by treatment with saponin at low concentration (50 micrograms/ml), while the function of sarcoplasmic reticulum (SR) was preserved. The strength of rigor contraction of the skinned fiber was used as a semi-quantitative index of the ability of SR to release Ca2+. When treated with saponin at high concentration (500 micrograms/ml), both sarcolemma and membrane of SR were skinned. Tension-pCa relationship and the Ca2+ concentration required for producing 50% of maximum Ca(2+)-activated tension (pCa50) were respectively taken as qualitative and quantitative index of Ca2+ sensitivity of troponin (TN). The results showed: (1) In low saponin skinned fiber, 5 and 10 mmol/L caffeine respectively produced 89.2 +/- 12.7 and 142.5 +/- 17 mg (n = 4) rigor contraction, 5 mmol/L theophylline did not have this effect. (2) In high saponin skinned fiber 5 mmol/L theophylline, 5 and 10 mmol/L caffeine shifted the tension-pCa relationship curve to the left, pCa50 respectively increased by 0.212, 0.261 and 0.274 pCa units. These results suggest that caffeine and theophylline increase the Ca2+ sensitivity of TN and caffeine can also induce Ca2+ release from SR.