Graduate School of Biomedical Sciences, Physiology
Myocardial Contraction; Myocardium; Heart; beta-Adrenergic Receptor Kinase; Phosphorylation; Rats, Sprague-Dawley; Academic Dissertations; Dissertations, UMMS
The purpose of this investigation was to examine selected biochemical mechanisms known to influence contractility and energy metabolism in the myocardium, with particular emphasis placed on the regulatory role of protein phosphorylation in the ventricular myocardium. The investigation was conducted in three phases; initially the cardiac contraction cycle was examined to determine whether reported fluctuations in myocardial cAMP levels were associated with other biochemical events known to be cAMP-dependent. The second phase involved the determination of specific kinase activities and endogenous substrates in a highly purified cardiac sarcolemmal preparation. In the final phase, ventricular myocytes were utilized to examine the ability of adenosinergic and muscarinic agonists to influence the isoproterenol-induced increases in protein phosphorylation.
Studies in the first phase examined cyclic AMP levels and selected kinase activities in hearts frozen at various stages of the cardiac cycle. An automated clamping device, capable of freezing a perfused rat heart in less than 50 msec, was utilized to separate the cardiac cycle into various phases. Three different timing schemes were employed to divide the cycle into 2 to 4 segments. These different timing schemes revealed no significant differences in cAMP during the cardiac cycle. Myocardial cAMP values ranged from 2.5 to 4.1 pmol/min/mg protein in all phases. However, in one scheme there was a tendency for cAMP to be elevated in early systole, with minimal values occurring diastole. There were also no significant differences seen for either glycogen phosphorylase or cAMP-dependent protein kinase (PKA) activity between various phases of the cardiac cycle. Since no significant fluctuations were observed in the levels of cAMP or the activities of PKA or glycogen phosphorylase during a single cardiac contraction cycle, it would appear that these agents do not exert their effects on cardiac function on a beat to beat basis.
The second phase of study examined the nature and function of individual protein kinases in the myocardium. Using a highly purified cardiac sarcolemmal preparation, kinase specific, synthetic substrates were employed to quantify the activities of cAMP-dependent (PKA), calcium/calmodulin-dependent (PKCM), calcium/phospholipid-dependent (PKC) and cGMP-dependent (PKG) protein kinases. Additionally, endogenous protein substrates were examined in this preparation to provide possible insight as to the function of these kinases in the heart. The activities of PKA, PKG, PKCM, and PKC in nmol 32P/min/μg protein were as follows: PKA, 1606; PKG, 35.7; PKCM, 353; and PKC, 13.2. Three endogenous protein substrates of apparent molecular weights of 15kD, 28kD and 92kD were phosphorylated. While no endogenous protein phosphorylation was detectable as a result of cG-PK activity, all of the substrates were phosphorylated, to varying degrees, by both PKA and CACM-PK. PKC phosphorylated only the 15kD substrate.
Even though several endogenous kinases are evident in the sarcolemmal preparation, cAMP-dependent protein kinase demonstrates the greatest degree of activity. This kinase also appeared to be the most abundant; however, there is some concern as to the source of these kinases in the membrane preparation since endothelial membranes as well as cardiac membranes appeared to be present. Evidence for endothelial contamination was provided by the finding that the membrane preparation contained appreciable amounts of angiotensin converting enzyme (ACE) activity, an enzyme felt to reside in the vascular endothelium. Since studies with this preparation could not exclude contribution of nonmuscle cell membranes a model consisting solely of dispersed ventricular myocytes was developed.
The third phase of these studies examined protein phosphorylation in primary cultures of ventricular myocytes. Specifically, these studies examined protein phosphorylation induced by exposure to isoproterenol (ISO), a catecholamine known to effect changes in the phosphorylation state of proteins in the heart by means of a β-adrenergic-mediated/cAMP-dependent mechanism was examined. Additionally, the effects of phenylisopropy-ladenosine (PIA) and carbamyl choline chloride (CARB) were examined with regard to their anti-adrenergic role(s) in this process.
Adherent, collagenase-dispersed, radiolabelled (32p) ventricular myocytes exposed to ISO demonstrated a dose and time dependent increase in 32p incorporation into several endogenous protein substrates. When the myocytes were exposed (60 sec) to either PIA or CARB prior to the exposure to ISO, ISO-induced 32p incorporation into protein substrates of apparent molecular weight of 6kD, 31kD and 155kD was reduced up to 67% when compared to the effects of ISO alone. Additionally, both PIA and CARB attenuated the ISO-induced increase in PKA activity in the myocyte, yet only CARB was seen to produce an inhibitory effect on the ISO-induced increase in cAMP levels in the myocytes. The effects of CARB were dose-dependent and inhibited the effects of ISO on 32p incorporation at all doses tested. PIA elicited biphasic effects: lower PIA concentrations were inhibitory in nature, while higher concentrations of PIA appeared to potentiate the increase in 32p incorporation induced by ISO. Based on electrophoretic mobilities (SDS/PAGE) of the 6kD and the 155kD substrates, these substrates have been tentatively identified as the monomeric form of the sarcoplasmic reticulum-associated protein, phospholamban, and the contractile filament-associated protein, C protein, respectively. The 31kD substrate has been identified, by means of immunoblot, as the contractile filament-associated protein, troponin I.
The role of protein phosphorylation in the myocardium involves complex, inter-related mechanisms that encompass extracellular, transmembranal and cytoplasmic elements in the heart. It is well understood that certain mechanisms of the contraction cycle known to vary on a beat to beat basis, such as myosin ATPase, involve changes in protein phosphorylation. However, the nature of the various kinases and substrates examined in this study appear to influence longer-term events of myocardial contractility. Mechanisms coupled with hormone action, modulation of second messenger-dependent components, and factors associated with changes in contractility seen with aging and disease are more likely to exhibit changes similar to those described herein. A better understanding of the underlying biochemistry may provide greater insight into the importance of these metabolic changes.
George, EE. Regulation of β-Adrenergic-Induced Protein Phosphorylation in the Myocardium: A Dissertation. (1990). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 228. https://escholarship.umassmed.edu/gsbs_diss/228
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