Cardiac function and phosphorylation of contractile proteins

Abstract

The primary regulation of cardiac contractility is probably through changes in the level of cytoplasmic free Ca 2+ . In the stimulation of contraction by catecholamines, secondary controls may be present at the level of the contractile proteins. Troponin-I, a subunit of the troponin complex of the thin filament, and C-protein, a thick filament component, are both phosphorylated in perfused hearts in response to catecholamines over time courses similar to that for the increase in contraction. Both proteins are also phosphorylated rapidly in vitro by cyclic-AMP-dependent protein kinase. Phosphorylation of troponin-I causes a decrease in the sensitivity of both cardiac myofibrillar ATPase and tension development of skinned fibre preparations to Ca 2+ , and also an increase in the rate of dissociation of Ca 2+ from isolated troponin. These results support the hypothesis that the role of phosphorylation of cardiac troponin-I is to contribute to the increased rate of relaxation of the heart that is observed with catecholamines. C-protein is phosphorylated to a maximum of 4-5 mol phosphate per mole protein both in vivo and in vitro . At present, however, the functions of both C-protein itself and its phosphorylation are unknown. Dephosphorylation of these contractile proteins after catecholamine stimulation is slow in perfused heart, although the rate can be increased by cholinergic agents. Phosphorylase, in contrast, is rapidly dephosphorylated under these circumstances. Phosphoprotein phosphatases relatively specific for phosphorylase have been identified in rat heart, whereas troponin-I appears to be dephosphorylated by general phosphatases. These observations account for the different rates of dephosphorylation of phosphorylase and the contractile proteins, but do not explain the slow dephosphorylation of the latter. In control perfused hearts myosin P-light chain was 50 % phosphorylated, and this was not changed by perfusion with positive inotropic agents or by short-term ischaemia. It was also unchanged during long-term hormonal modifications. Perfusions with 32 P 1 in rat heart give a half-time for the turnover of phosphate bound to the P-light chain of 2-4 min, showing that the myosin light chain kinase and phosphatase are active in the heart. It is hypothesized that under control conditions the kinase is already fully active, and that an increase in cytoplasmic Ca 2+ cannot therefore cause further activation of the enzyme.

Tumor-promoting phorbol esters inhibit cardiac functions and induce redistribution of protein kinase C in perfused beating rat heart.

“…Phosphorylation of troponin I is known to decrease the affinity of troponin complex for Ca 2+ . 24 - 25 It has also been found that C protein of myocardial thick filament was phosphorylated by protein kinase C, 26 and that this phosphorylated C protein had a decreased effect in stimulating act in-activated myosin ATPase activity. 27 However, since the native protein kinase C (M r 78,000) is activated only in the presence of phospholipid, it may not be functional in the cytosol.…”

Differential roles of regulatory light chain and myosin binding protein-C phosphorylations in the modulation of cardiac force development

“…The rates of cross-bridge attachment and detachment, however, are finely tuned in order to optimize cardiac performance on a beat-to-beat basis. The cardiac isoforms of the myofilament proteins, myosin binding protein-C (cMyBP-C) and troponin I (cTnI), are phosphorylated rapidly by protein kinase A J Physiol 588.6 (PKA) over time courses similar to that for the increase in twitch force and kinetics during β 1 -adrenergic stimulation (England, 1983). In contrast, under physiological conditions the level of regulatory light chain (RLC) phosphorylation is maintained relatively constant by a balance of phosphorylation and phosphatase-induced dephosphorylation, and is regulated by α 1 -adrenergic tone (Chan et al 2008).…”

Structure of C protein purified from cardiac muscle.

“…It has been suggested that the physiological role of C protein may involve a calcium-regulated binding of C protein to thin filaments, because C protein binding to native thin filaments occurs only in the presence of micromolar concentrations of calcium (20). Recently, it has been shown that C protein in cardiac muscle becomes phosphorylated in re-sponse to B-adrenergic agonists and dephosphorylated in response to cholinergic agonists (5,11,12). The level of C protein phosphorylation correlates with the rate of relaxation of the cardiac contraction and it has been suggested that C protein regulates twitch relaxation in cardiac muscle (9).…”