Biochemical basis of heart function. 1. Relation of catecholamine stores and contractile force in an isolated rat heart

Catecholamine effect on HCO3-/Cl- exchange and rabbit myocardial cell pH regulation

The possible effect of isoproterenol on a HCO3-/Cl- exchange in mammalian myocardium and its role on intracellular pH (pHi) regulation was studied in isolated perfused rabbit hearts. pHi was determined from the distribution of 5.5-dimethyl-2,4-oxazolidinedione (DMO). Perfusates contained either 137.4 or 4.4 mM Cl-. In the latter case, Cl- was replaced by glucuronate. Hearts were perfused with low [Cl-] Ringer for 1 h in order to deplete the cell of Cl-. Isoproterenol (10(-5) M) was infused for the last 15 min of perfusion in approximately half of the preparations. Reduction of [Cl-], both in the untreated and in the isoproterenol-treated preparations resulted in an increase in pHi. Isoproterenol increased tissue cAMP concentration by approximately 2.5 fold both in hearts perfused with normal and with low [Cl-] Ringer. In the presence of normal [Cl-], isoproterenol had an alkalinizing effect when acid loads were introduced. This effect was abolished by the reduction of [Cl-]. These results support the idea that the catecholamine-cAMP system stimulates a HCO3-/Cl- exchange in mammalian myocardium.

Platelet alpha 2 adrenoreceptors in chronic congestive heart failure

Patients with chronic congestive heart failure (CHF) are known to have elevated plasma concentrations of norepinephrine. Although this elevation of catecholamines in plasma may facilitate myocardial contractility, it may also be toxic to the myocardium in the long term. The alpha 2 adrenoreceptor located on noradrenergic nerve terminals regulates neuronal norepinephrine release by feedback inhibition. This receptor is also located on human blood platelets. This study determines the status of platelet alpha 2 adrenoreceptors in 16 patients with CHF (class I and II in 7 and class III and IV in 9) and in 26 normal volunteers. Specific high-affinity binding of the alpha 2 agonist 3H-clonidine and the alpha 2 antagonist 3H-yohimbine was used to determine the number (Bmax) of alpha 2 receptors and the dissociation constant (KD) for the 2 ligands. In the control population, the Bmax (in fmol/mg protein) for 3H-clonidine was 33 +/- 2 and for 3H-yohimbine was 165 +/- 12. There was a 25% difference in the maximum number of specific binding sites for 3H-clonidine in the class III/IV group (Bmax 24 +/- 2, p less than 0.05) and a 43% difference in the maximum number of specific binding sites for 3H-yohimbine (Bmax 94 +/- 9; p less than 0.005). There was a smaller but nonsignificant difference in the number of receptors on platelets from patients in the class I and II group. The KD's were similar in all 3 groups. These differences correlated well with the increases in plasma norepinephrine levels between the normal group (273.8 +/- 44.1 pg/ml) and the class III/IV group (1333.5 +/- 244.9, p less than 0.0005). This study supports the hypothesis that increased levels of circulating norepinephrine in CHF lead to a decrease in platelet alpha 2 adrenoreceptors. Further studies should be performed to determine whether pharmacologic stimulation of these receptors might lead to a decrease in the neuronal release of that norepinephrine which might be toxic to the myocardium. Monitoring of platelet alpha 2 adrenoreceptor number may provide a guide to therapy of CHF.

Increased number of beta-adrenergic receptors in the hypertrophied myocardium

Development of cardiac hypertrophy is associated with depletion of endogenous catecholamine stores and increased inotropic response to exogenous catecholamines. A biochemical basis for these changes is provided by the observation that the number of cardiac beta-adrenergic receptors - as reflected in specific [3H]dihydroalprenolol binding - is increased in hypertrophy without a change in the affinity of dihydroalprenolol for the binding sites or in the capacity of isoproterenol to displace dihydroalprenolol. This change in beta-receptor numbers may be an important adaptive mechanism for preserving the contractile performance of the hypertrophied myocardium.

[The Contractile Function of the Surviving Heart Muscle Following Coronary Occlusion (author's transl)]

Following acute myocardial infarction the functional load of the surviving heart muscle does increase considerably, leading to an increased release of adrenergic neurotransmitters with a consequent decrease in endogenous catecholamine stores. Within the first 24 h following infarction, a temporary decline in the high-energy phosphate content is observed in the surviving heart muscle; futhermore, a reduction in lactate extraction is noted. In the intact organism an increased shortening of the surviving heart muscle is noted as a consequence of the altered ventricular geometry and the increased release of catecholamines. If these effects are excluded by means of isolation and analysis in vitro, a decrease in contractile function could be demonstrated in the surviving heart muscle in the early phase following infarction; the response to positive inotropic interventions was depressed as well. These changes are reversible; six weeks following infarction a normal contractile behaviour is observed.

Experimental myocardial infarction in the cat. I. Reversible decline in contractility of noninfarcted muscle

The contractile state of the noninfarcted myocardium was examined in adult cats after myocardial infarction produced by ligation of several branches of the left coronary artery. At 2 days, 7 days, and 6 weeks after infarction, and after determination of intracardiac pressures, papillary muscles were exicised from the noninfarcted segment of the right ventricle and attached to a myograph for analysis of contractile function. One week after infarction there was a decline in actively developed force at Lmax, caused by a decrease in the rate of force development. In addition, the response to procedures that augment myocardial contractility, such as paired stimulation and increasing the frequency of electrical stimulation, was significantly depressed. Two days after infarction, changes were less significant, although similar in direction. Six weeks after infarction, developed force at Lmax had returned to normal values. The response to procedures augmenting contractility also had returned to normal. There appears to be a distinct, reversible loss of contractility in the remaining viable myocardium in the early phase after experimental infarction.

Alterations in myocardial function during bacterial infective cardiomyopathy

The status of myocardial function in rabbits subjected to cardiac catheterization and infection with Streptococcus viridans was assessed at 3 and 6 days. Sham-operated control animals as well as uninfected catheterized animals were used for comparison. Although left heart hypertrophy and interstitial edema were evident in both uninfected and infected animals, the infected animals exhibited in addition mononuclear cell infiltration and muscle degeneration as well as lung congestion and accumulation of pleural fluid. Both uninfected and infected animals has elevated levels of serum creatine phosphokinase, lactic dehydrogenase and glutamic oxaloacetic transaminase as well as electrocardiographic abnormalities such as increased amplitude of the ORS complex and flattening or inversion of the T wave. Unlike findings in the uninfected animals, the serum calcium, magnesium and sodium levels were slightly but significantly decreased and serum potassium levels were increased in the infected rabbits. Both heart rate and pulse pressure were higher in 6 day uninfected and 3 day infected animals whereas 6 day infected animals showed a decrease in heart rate. In comparison to the sham-operated control rabbits and the uninfected animals, the infected animals exhibited depression in the rates of left ventricular pressure development and relaxation as well as prolongation in time for half relaxation in situ. Relative maximal contractile element velocity extrapolated from intraventricular pressure-velocity curves was decreased by 24, 52 and 76 percent, respectively, of control values in the uninfected hearts and those with 3 and 6 days of infection. The isolated perfused hearts from infected animals also generated less contractile force and showed a decrease in the rates of contraction and relaxation, but half-relaxation time was increased. These results demonstrate myocardial dysfunction during experimental bacterial endocarditis and provide evidence that infective cardiomyopathy is associated with heart failure.

Postnatal changes in rat ventricular function

Experiments were designed to assess the intrinsic changes in myocardial function with age and to determine the role of increasing left ventricular mass in this process. Isometric ventricular pressure-volume curves and length-tension curves were measured in isolated perfused rat hearts of different ages and masses. Incremental tension, peak active tension, and peak passive tension were less for 10-day-old hearts than they were for older hearts subjected to proportionately equal increases in length. At resting length, peak active tension developed by hearts at least 16 days old was 78% higher than that developed by 10-day-old hearts, and peak passive tension was 133% higher. There was no significant difference between 16-day-old rats and older rats. Thus, by the sixteenth postnatal day, the heart had developed functional maturity. These functional changes appeared to be related to age rather than to left ventricular mass. Histological studies of the left ventricle showed that, at birth, the myofibrils were disoriented with a high ratio of nonmyofibrillar elements to myofibrillar elements. Within 16-20 days after birth, the structural organization and the ratio of nonmyofibrillar elements to myofibrillar elements were similar to the organization and the ratio found in an adult. Thus a direct relationship existed between the functional properties and the anatomical development of the myocardium. Mass per se was not the sole factor that contributed to improved myocardial function with age.