Role of Ca(2+)-calmodulin dependent phospholamban phosphorylation on the relaxant effect of beta-adrenergic agonists

The role of the Ca(2+)-calmodulin dependent pathway of phospholamban phosphorylation on the relaxant effect of beta-adrenergic agonists was studied in isolated perfused rat heart. Administration of the calmodulin antagonist W7 or lowering [Ca]o from 1.35 mM (control) to 0.25 mM, were used as experimental tools to inhibit the Ca(2+)-calmodulin dependent protein kinase activity. 3 x 10(-8) M isoproterenol increased cAMP levels from 0.613 +/- 0.109 pmol/mg wet weight to 1.581 +/- 0.123, phospholamban phosphorylation from 36 +/- 6 pmol 32P/mg protein to 277 +/- 26 and decreased time to half relaxation (t1/2) from 61 +/- 2 msec to 39 +/- 2. Simultaneous perfusion of isoproterenol with 10(-6) M W7, decreased phospholamban phosphorylation to 170 +/- 23 and prolongated t1/2 to 47 +/- 3 but did not affect the increase either in cAMP levels or myocardial contractility produced by isoproterenol. Similar effects on phospholamban phosphorylation and myocardial relaxation were obtained when isoproterenol was perfused in low [Ca]o. Low [Ca]o did not affect the increase in cAMP elicited by isoproterenol but offset the positive inotropic effect of the beta-agonist. The results suggest a physiological role of the Ca(2+)-calmodulin dependent phospholamban phosphorylation pathway as a mechanism that supports, in part, the beta-adrenergic cardiac relaxant effect.

Characteristics of ryanodine-induced tetani in the perfused rat heart. Tetanic tension is not the highest force that cardiac muscle can generate

The aim of the present study was to elucidate the conditions required to obtain tetanic contractions in rat intact heart and to investigate whether tetanic tension was actually the maximal tension that isolated rat heart is able to generate. Experiments were performed on isolated rat hearts (Langendorff technique) perfused at constant coronary flow (8-9 ml/min). Rapid repetitive stimulation (400 to 3000 pulses/min) failed to elicit a fused tetanus. The first twitch that occurred at the end of the rapid stimulation period was a potentiated beat (PSP) of significantly greater magnitude than that of the regular twitch. This potentiation declined in successive beats. When rapid electrical stimulation (600 to 3000 pulses/min) was applied to hearts treated with 5 x 10(-6) M ryanodine, the result was a fused and steady tetanic tension. Ryanodine suppressed PSP. Tetanic tension could be graded by stepwise increase of [Ca2+]o from 0.25 to 5 mM. Maximal tetanic tension occurred at a [Ca2+]o between 3.85 and 5 mM. At any of the [Ca2+]o, tetanic tension was significantly greater than the tension of the twitch obtained at approximately the natural frequency of rat heart in the intact animal (250 beats/min) but it did not differ significantly from the twitch obtained at 100 beats/min. Moreover, the tension of PSP at 0.25 and 1.35 mM [Ca2+]o was significantly greater than the maximal tetanic tension that could be obtained. Similar results to that obtained with ryanodine, were obtained in additional experiments in which caffeine was used to evoke tetanic contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of phospholamban in the intact heart. A study on the physiological role of the Ca(2+)-calmodulin-dependent protein kinase system

The aim of the present study was to further elucidate the physiological role of the calcium-calmodulin (Ca(2+)-Cm)-dependent protein kinase system on phospholamban phosphorylation in the intact functioning heart. The effect of increasing extracellular calcium concentration [Ca]o on phospholamban phosphorylation (PHPL) was studied under different experimental conditions: (a) regular twitches and ryanodine induced-tetani both in the presence and in the absence of 3 x 10(-8) M isoproterenol and (b) Post-stimulation potentiation (PSP), i.e. the potentiation of contractility that follows a period of rapid repetitive stimulation. In the regular twitch, the increase in [Ca]o enhanced contractility both, in the absence and in the presence of beta-stimulation without changing basal or isoproterenol stimulated cAMP levels respectively. This increase in contractility was accompanied by a significant enhancement of PHPL-from 90.6 +/- 16.4 to 216 +/- 35.2 pmols 32Pi/mg protein at 0.25 and 3.85 mM [Ca]o respectively-only when isoproterenol was present. The calmodulin antagonist W-7 significantly decreased the isoproterenol-induced phosphorylation of phospholamban at [Ca]o 1.35 mM. Similar results were obtained under tetanic conditions. When myocardial contractility was enhanced by PSP up to ten-times with respect to the regular twitch, no detectable effect in PHPL was observed. Indirect evidence obtained from skinned rat cardiac trabeculae suggested that the failure of the cAMP-independent mechanisms to phosphorylate phospholamban is not related to a deficient increase in intracellular calcium. The results support the notion that the increase in intracellular calcium induces an increase in PHPL only at high intracellular cAMP levels.

Decrease in tetanic tension elicited by beta-adrenergic stimulation

The effect of beta-adrenergic stimulation on tetanic tension (TT), maximal rate of rise of tension (+TT) and phospholamban (PHL) phosphorylation were studied in the perfused rat heart. 3 x 10(-8) M isoproterenol perfused at different [Ca2+]o 0.25, 1.35 and 3.85 mM, significantly decreased TT while increased +TT and PHL phosphorylation at the three [Ca2+]o studied. Regression lines of the relationship between +TT and TT from individual data obtained at each [Ca2+]o in the presence and in the absence of isoproterenol, show that for the same level of +TT, TT is lower in the presence of isoproterenol, i.e. at high levels of PHL phosphorylation. The slopes of the lines were 0.137 s and 0.427 s (P less than 0.05) in the presence and absence of isoproterenol respectively. The decrease in TT produced by the beta-agonist can be attributed to its relaxant action prevailing over its inotropic effect and may represent the mechanical expression of the enhanced phosphorylation of phospholamban.

cAMP and calcium-dependent mechanisms of phospholamban phosphorylation in intact hearts

The present work was undertaken with two main goals: 1) to further elucidate the physiological role of the adenosine 3',5'-cyclic monophosphate (cAMP) and Ca2(+)-calmodulin (Ca2(+)-Cm)-dependent mechanisms of phospholamban phosphorylation (32PiPHL), and 2) to study the possible interaction between these two systems in the intact heart. Interventions that increased twitch or tetanic tension without modifying cAMP levels [high extracellular Ca2+ concentration [( Ca2+]o) or BAY K 8644 in catecholamine-depleted hearts] failed to alter 32PiPHL. Moderate and high beta-adrenergic stimulation (3 x 10(-9) and 3 x 10(-8) M isoproterenol, respectively) increased cAMP from 0.345 +/- 0.032 to 0.636 +/- 0.069 and 0.772 +/- 0.060 pmol/mg wet wt, and 32PiPHL from 26.8 +/- 4.1 to 58.6 +/- 13.1 and 174.7 +/- 13.8 pmol 32Pi/mg sarcoplasmic reticular [SR] protein, respectively. Both doses of isoproterenol produced an enhanced myocardial relaxation. Reversal of the positive inotropic effect of isoproterenol by interventions that decrease intracellular Ca2+ supply failed to reduce the enhancement in 32PiPHL and myocardial relaxation elicited by 3 x 10(-9) M isoproterenol but diminished the increase in 32PiPHL induced by 3 x 10(-8) M isoproterenol to 116.3 +/- 10.9 without significant changes in cAMP. Changes in myocardial relaxation closely paralleled the changes in 32PiPHL. These results suggest that 1) 32PiPHL may be enhanced by the cAMP-dependent mechanism independently of the Ca2(+)-Cm system, and 2) 32PiPHL and myocardial relaxation may be modified by intracellular Ca2+ changes only at high-intracellular cAMP levels.

The effects of Bay K 8644 on myocardial relaxation and cAMP levels in perfused rat heart: role of sympathetic neurotransmitter release

Bay K 8644 typifies a number of drugs known to act directly on voltage-dependent calcium channels to increase calcium current. Such effects probably underly the drug's positive inotropic action and smooth muscle stimulation. Although the effects of this compound on myocardial contractility have been extensively described, its action upon myocardial relaxation is not well established. Either no changes or a prolongation in ventricular relaxation have been mentioned. On the other hand, during the course of other experiments performed in our laboratory with the perfused rat heart (unpublished results), we observed that Bay K 8644 elicits a moderate but consistent relaxant effect. The present work was undertaken in an attempt to clarify the effect of Bay K 8644 upon myocardial relaxation. Evidence will be presented showing that in the perfused rat heart, the positive inotropic action of Bay K 8644 occurs together with a prolongation of the contraction time (TTP) without changes in time to half relaxation (t 1/2). However, an enhancement of ventricular relaxation was detected by the proportional greater increase in maximal velocity of relaxation (-T) with respect to maximal velocity of contraction (+T) and the shortening of the time constant of relaxation (tau). These actions occur associated with a significant increase in cAMP levels and phospholamban phosphorylation. Either the relaxant effect as well as the increments in cAMP and phospholamban phosphorylation were abolished when the hearts were depleted of norepinephrine by previous treatment with reserpine. Depletion of norepinephrine stores also decreases the positive inotropic effect of the drug.(ABSTRACT TRUNCATED AT 250 WORDS)