Adenylate cyclase activity coupled to the stimulatory guanine nucleotide binding protein in patients having electrophysiologic studies and either structurally normal hearts or idiopathic myocardial disease

Differential effects of ovariectomy on calcium activation of cardiac and soleus myofilaments

The hypothesis that ovarian sex hormone deficiency affects cardiac myofilament activation was tested. Chemically skinned ventricular trabeculae and single soleus muscle fibers were prepared from 10- and 14-wk ovariectomized and control rats. Tension-pCa (-log [Ca(2+)]) relations of left ventricular trabeculae and soleus fibers were compared to test whether thin filament proteins are potential sites of modulated activation. Trabeculae from ovariectomized rats exhibited a significant increase in Ca(2+) sensitivity with no change in maximal tension-generating ability. In contrast, soleus fibers demonstrated no shift in Ca(2+) sensitivity but generated significantly less maximal tension. No changes in thin filament protein isoform expression or loss of thin filament proteins were apparent in the trabeculae or soleus fibers from ovariectomized rats. Although not directly tested, our results are consistent with a possible modulation of regulatory proteins (e.g., cardiac troponin I) to account for the observed change in myofilament responsiveness of hearts from ovariectomized rats. Other possible mechanisms for the altered myocardial Ca(2+) sensitivity after ovariectomy are discussed.

Troponin I phosphorylation in the normal and failing adult human heart

BACKGROUND

In the failing human heart myofibrillar calcium sensitivity of tension development is greater and maximal myofibrillar ATPase activity is less than in the normal heart. Phosphorylation of the cardiac troponin I (cTnI)-specific NH2-terminus decreases myofilament sensitivity to calcium, while phosphorylation of other cTnI sites decreases maximal myofibrillar ATPase activity.

METHODS AND RESULTS

We examined cTnI phosphorylation in left ventricular myocardium collected from failing hearts at the time of transplant (n=20) and normal hearts from trauma victims (n=24). The relative amounts of actin, tropomyosin, and TnI did not differ between failing and normal myocardium. Using Western blot analysis with a monoclonal antibody (MAb) that recognizes the striated muscle TnI isoforms, we confirmed that the adult human heart expresses only cTnI. A cTnI-specific MAb recognized two bands of cTnI, designated cTnI1 and cTnI2, while a MAb whose epitope is located in the cTnI-specific NH2-terminus recognized only cTnI1. Alkaline phosphatase decreased the relative amount of cTnl1, while protein kinase A and protein kinase C increased cTnI1. The percentage of cTnI made up of cTnI1, the phosphorylated form of TnI, is greater in the normal than the failing human heart (P<.00).

CONCLUSIONS

This phosphorylation difference could underlie the reported greater myofibrillar calcium sensitivity of failing myocardium. The functional consequence of this difference may be an adaptive or maladaptive response to the lower and longer calcium concentration transient of the failing heart, eg, enhancing force development or producing ventricular diastolic dysfunction.

Uncoupling of beta 1-adrenoceptors from cardiac adenylyl cyclase in cardiomyopathic and control hamsters

The beta-adrenoceptor-adenylyl cyclase system was studied in heart ventricles from Wistar rats, cardiomyopathic (BIO 8262) and nonfailing control hamsters (CLAC) using the beta 1-adrenoceptor antagonist CGP 20712A. In radioligand binding studies, the majority of beta-adrenoceptors in ventricles from rats as well as from CLAC hamsters was of the beta 1-subtype (72.2% and 76.6%, respectively). In BIO ventricles a significant (CLAC vs. BIO, P < 0.05) reduction in the beta 1-subtype (62.9%) was found. In Wistar rats the subtype-mediated stimulation of adenylyl cyclase reflected the beta 1:beta 2 ratio as determined by binding studies. In hamster ventricles the effect of isoprenaline was mediated predominantly (CLAC) or exclusively (BIO) via the beta 2-subtype, indicating that cardiac beta 1-adrenoceptors were partly (CLAC) or completely (BIO) uncoupled from the adenylyl cyclase.

Beta-adrenergic neuroeffector abnormalities in the failing human heart are produced by local rather than systemic mechanisms

In order to investigate the general cause of beta-adrenergic receptor neuroeffector abnormalities in the failing human heart, we measured ventricular myocardial adrenergic receptors, adrenergic neurotransmitters, and beta-adrenergic receptor-effector responses in nonfailing and failing hearts taken from nonfailing organ donors, subjects with endstage biventricular failure due to idiopathic dilated cardiomyopathy (IDC), and subjects with primary pulmonary hypertension (PPH) who exhibited isolated right ventricular failure. Relative to nonfailing PPH left ventricles, failing PPH right ventricles exhibited (a) markedly decreased beta 1-adrenergic receptor density, (b) marked depletion of tissue norepinephrine and neuropeptide Y, (c) decreased adenylate cyclase stimulation in response to the beta agonists isoproterenol and zinterol, and (d) decreased adenylate cyclase stimulation in response to Gpp(NH)p and forskolin. These abnormalities were directionally similar to, but generally more pronounced than, corresponding findings in failing IDC right ventricles, whereas values for these parameters in nonfailing left ventricles of PPH subjects were similar to values in the nonfailing left ventricles of organ donors. Additionally, relative to paired nonfailing PPH left ventricles and nonfailing right ventricles from organ donors, failing right ventricles from PPH subjects exhibited decreased adenylate cyclase stimulation by MnCl2. These data indicate that: (a) Adrenergic neuroeffector abnormalities present in the failing human heart are due to local mechanisms; systemic processes do not produce beta-adrenergic neuroeffector abnormalities. (b) Pressure-overloaded failing right ventricles of PPH subjects exhibit decreased activity of the catalytic subunit of adenylate cyclase, an abnormality not previously described in the failing human heart.

Signal transduction by G proteins in cardiac tissues

The role of G proteins in mediating the responses of the heart to circulating catecholamines and to the influences of the autonomic nervous system is of special interest to cardiologists. It is evident that G proteins are essential links in the cascade of biochemical events that ensure when neurotransmitters and hormones interact with receptors on myocardial cells. It is likely [corrected] that dysfunction of G proteins plays a role in cardiovascular pathophysiology. With current methodologies, especially molecular biological and recombinant DNA techniques, and with transgenic animal models that can relate physiological function and specific gene dosage, some cardiovascular diseases may be traced to G protein-related defects.

Immunodetectable levels of the inhibitory guanine nucleotide-binding regulatory proteins in failing human heart: discordance with measurements of adenylate cyclase activity and levels of pertussis toxin substrate

Human hearts with idiopathic dilated cardiomyopathy have diminished adenylate cyclase activity and increased amounts of the alpha-subunit of the inhibitory guanine nucleotide-binding regulatory protein (alpha Gi) as measured by pertussis toxin catalyzed ADP-ribosylation. We utilized specific antisera against synthetic peptides corresponding to amino sequences deduced from cDNA's encoding the three alpha Gi subspecies to compare the immunologic and bioactivity levels of Gi in failing and non-failing human hearts. The various antisera detected three peptides with Mr 42,000, 38,000, and 37,000. Only the Mr 42,000 peptide co-migrated with the pertussis toxin substrate. Although functional activity of alpha Gi was increased in the particulate fractions of the failing heart as measured by inhibition of guanine nucleotide-stimulated adenylate cyclase activity and the quantity of pertussis toxin substrate was also increased, there were not associated changes in the levels of immunodetectable Gi. Therefore, the increased functional activity of Gi in the failing human heart as assessed by adenylate cyclase measurements cannot be explained by a relative increase in the among of Gi protein.

A1-adenosine receptor inhibition of adenylate cyclase in failing and nonfailing human ventricular myocardium

BACKGROUND

Receptors that couple via the stimulatory G protein, Gs, to adenylate cyclase and to a positive inotropic response have been extensively investigated in falling human heart. In contrast, much less is known about receptors, such as the A1-adenosine receptor, that couple to adenylate cyclase via the inhibitory G protein, Gi, to give a negative inotropic response. Activation of such Gi-coupled receptors might worsen heart failure. Furthermore, alpha Gi is increased in failing human ventricular myocardium, which may enhance inhibitory receptor coupling to adenylate cyclase.

METHODS AND RESULTS

A1-Adenosine receptor inhibition of adenylate cyclase was examined in crude particulate preparations derived from 12 nonfailing and 12 failing human left ventricles. Experimental conditions were designed for maximal inhibitory responses. Dose-response curves were performed with the selective A1-adenosine receptor agonist R-phenylisopropyl-adenosine (R-PIA). No differences in nonfailing versus failing heart were observed for basal adenylate cyclase activity (49.0 +/- 4.1 versus 45.7 +/- 2.6 pmol cyclic AMP/min/mg), maximal R-PIA-mediated inhibition (31.1 +/- 2.6 versus 30.2 +/- 1.6 pmol cyclic AMP/min/mg), ED50 (R-PIA x 10(-7) 1.28 +/- 0.10 versus 1.36 +/- 0.08), or slope (1.06 +/- 0.06 versus 1.03 +/- 0.10), respectively. Furthermore, fluoride, forskolin, and manganese adenylate cyclase activation were not different in failing heart, which is consistent with no change in the catalytic unit of adenylate cyclase. The inhibitory G protein alpha Gi, as quantitated by pertussis toxin-catalyzed ADP-ribosylation, was increased in failing heart (105.7 +/- 5.8 versus 132.7 +/- 3.4 optical density units, p less than 0.003). Basal adenylate cyclase activity was reduced in failing heart (7.8 +/- 0.8 versus 4.5 +/- 0.4 pmol cyclic AMP/min/mg, p less than 0.005) with assay conditions designed to assess G protein effects.

CONCLUSIONS

The A1-adenosine receptor pathway exerts a major inhibitory effect on human myocardial adenylate cyclase activity. Although alpha Gi was increased in failing heart, A1-adenosine receptor inhibition of adenylate cyclase was not altered in preparations of failing versus nonfailing human ventricular myocardium.

Cardiovascular responses to milrinone in pertussis toxin-pretreated pithed rats

The modulating effects of pertussis toxin on angiotensin II and B-HT 920-evoked hemodynamic changes were compared with those of milrinone to evaluate the possible role of guanine nucleotide regulatory proteins (G proteins) in the mechanism of action of milrinone. Both milrinone and pertussis toxin shifted the blood pressure dose-response curves of B-HT 920 to the right, but the responses to angiotensin II were decreased after milrinone pretreatment only. The increase in cardiac frequency evoked by milrinone and isobutylmethylxanthine (IBMX) was not sensitive to pertussis toxin. In contrast, the decrease in systolic blood pressure elicited by milrinone could be prevented by pertussis toxin pretreatment, suggesting the involvement of a regulatory protein. Milrinone and IBMX did not influence the effects of arecoline on blood pressure or heart rate in either normal or pertussis toxin-pretreated rats. It is concluded that milrinone may affect a G protein, but not the adenylate cyclase-associated inhibitory protein, Gi.