Mechanics of cardiac contraction and the phosphorylation of sarcotubular and myofilament proteins. In: Ter Keurs HEDJ, Tyberg JV, editors. Mechanics of the Circulation. Dordrecht: Springer Netherlands; 1987. pp. 135-52. [DOI: 10.1007/978-94-009-3311-8_12]

Sarcolemma, sarcoplasmic reticulum, and sarcomeres as limiting factors in force production in rat heart

Inotropic interventions were compared with respect to their maximum effect on force of contraction in rat myocardium to identify limiting steps in calcium handling. Peak force, sarcomere length, and action potentials were measured in thin ventricular trabeculae. Relevant control conditions were stimulation frequency, 0.2 Hz; [Ca2+]o, 1 mM; [K+]o, 5 mM; [Na+]o, 150 mM. The inotropic interventions and results were as follows. 1) The interventions of high [Ca2+]o, low [Na+]o, high [K+]o, addition of tetraethylammonium chloride, or postextrasystolic potentiation resulted in approximately the same (within 5%) maximum force (Fmax). Above the respective optimum doses, force declined and aftercontractions were often observed. Combinations of the different interventions never enhanced force to above Fmax. This suggests that Fmax is determined by a maximum level of Ca2+ in the sarcoplasmic reticulum, above which spontaneous release occurs. 2) Sr2+ (10 mM) caused an increase of force to 1.3 X Fmax and lengthening of contraction and action potentials. The force-sarcomere length relation was, then, similar to that in skinned fibers at maximum activation. Hence, 1.3 X Fmax reflects saturation of the sarcomeres. We postulate that a large influx of Sr2+ during the long action potential can circumvent the reticulum and activate the sarcomeres directly. When the reticulum was blocked with ryanodine, maximum force of tetanic contractions was about 1.1 X Fmax. This result supports the above conclusions. 3) Isoproterenol increased force to a maximum that was 20% below Fmax and shortened the contraction. This may be due to a decreased sensitivity of the sarcomeres to Ca2+ or to stimulation of the Ca2+ pump in the reticulum, that is, an increasing fraction of the released Ca2+ is sequestered before it can activate the sarcomeres. Thus, three factors that limit force production were identified, depending on the inotropic stimulus.

Inotropic responses to isoproterenol and phosphodiesterase inhibitors in intact guinea pig hearts: comparison of cyclic AMP levels and phosphorylation of sarcoplasmic reticulum and myofibrillar proteins

The influence of selective (milrinone: 10, 50, 100 microM) and nonselective phosphodiesterase (isobutylmethylxanthine: 0.1, 10, 100 microM) inhibitors and beta-adrenergic stimulation (isoproterenol: 0.01, 0.1 microM) on phospholamban and myofibrillar protein phosphorylation was studied in guinea pig hearts perfused with [32P]orthophosphate. Changes in protein phosphorylation were compared to alterations in tissue cyclic AMP (cAMP) levels and positive inotropic effects induced by these agents. Isoproterenol (0.01 microM), milrinone (50 microM), and isobutylmethylxanthine (100 microM) all produced similar, twofold increases in dP/dt and -dP/dt but only stimulation with isobutylmethylxanthine and isoproterenol was associated with significant increases in phospholamban phosphorylation. At these equipotent doses, the effects of isobutylmethylxanthine were associated with higher increases (3.1-fold) in cAMP than those observed with isoproterenol (twofold). Milrinone (50 microM) produced a 2.5-fold increase in cAMP levels but failed to change phospholamban phosphorylation. Higher doses of milrinone (100 microM) resulted in relatively high (4.1-fold) cAMP levels, and this was associated with increased (1.5-fold) phosphorylation of phospholamban. Phosphorylation of troponin I was significantly increased at 0.01 microM and 0.1 microM isoproterenol, while phosphorylation of C protein was observed only at 0.1 microM isoproterenol. Isobutylmethylxanthine and milrinone did not significantly increase phosphorylation of either troponin I or C protein at any of the doses studied. These findings indicate that cardiotonic agents acting via the cAMP pathway may produce similar inotropic responses at different levels of cAMP and phosphorylation of sarcoplasmic reticulum and myofibrillar proteins.