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Expression and pharmacological characterization of a stimulatory subtype of adenosine receptor in fetal chick
1Department of Medicine (Cardiology), University of Pennsylvania School of Medicine, Philadelphia.
Abstract:
Ventricular and atrial myocytes cultured from chick embryos 14 days in ovo were used as model systems to study cardiac adenosine receptors. In membranes of ventricular cultures, blocking of the A1-adenosine receptor pathway by the A1-selective antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) or by pertussis toxin treatment of the myocyte resulted in a significant adenosine agonist-mediated stimulation of the adenylate cyclase activity. The maximal increases in adenylate cyclase activity caused by the equipotent or the A2-adenosine receptor-selective agonists (from 52.1 +/- 3% to 63 +/- 10% [mean +/- SEM]) were significantly greater than those caused by the A1-selective agonists (from 11 +/- 5% to 34.6 +/- 7%) (p less than 0.01, by t test, n = 4-8). However, in membranes of atrial myocytes, when A1-subtype had been blocked, the various adenosine agonists had no effect on the adenylate cyclase activity. Whether the stimulatory adenylate cyclase-coupled adenosine receptor is also capable of stimulating contractility in the intact ventricular myocyte was next investigated. In ventricular but not in atrial cells, the various adenosine agonists caused an increase in the contractile amplitude in the presence of DPCPX or in myocytes preexposed to pertussis toxin. The increase in contraction amplitude caused by each agonist was expressed as percent of maximum (maximum is the increase in contractility caused by 2.4 mM calcium). In the pertussis toxin-treated myocyte, the maximal increases caused by the equipotent or A2-agonists (NECA, MECA, CV-1808, and CGS21680, from 49.6 +/- 3% to 52.5 +/- 6%, n = 8-12) were significantly greater than those elicited by the A1-agonists (2-CADO, S-PIA, R-PIA, and DCCA, from 12 +/- 4% to 37 +/- 3%, n = 8) (p less than 0.05, by t test). These data demonstrated that a stimulatory adenosine receptor, likely the A2-adenosine receptor, was present on the ventricular but not the atrial myocytes and was linked directly to a stimulation of the cardiac contractility. The functional effects mediated by the A1-subtype became manifested in the presence of isoproterenol, as evidence by an inhibition of the isoproterenol-stimulated increases in adenylate cyclase activity and in cardiac contractility by adenosine agonists. Thus, both subtypes of adenosine receptors, each mediating opposing responses, were present on the ventricular myocytes, whereas only the A1-subtype was found in the atria. The presence of a stimulatory functional A2-adenosine receptor may help explain the absence of a direct negative inotropic response to adenosine in the ventricle.
Insights
Chick embryo heart cells reveal that A2-adenosine receptors stimulate contractility in ventricles but not atria. This finding helps explain adenosine's varied effects on heart muscle contraction.
Area of Science:
- Cardiology
- Pharmacology
- Cell Biology
Background:
- Adenosine receptors play crucial roles in cardiovascular function.
- Cardiac myocytes express different adenosine receptor subtypes (A1 and A2) that mediate opposing effects.
- Understanding the distribution and function of these receptors is key to elucidating cardiac regulation.
Purpose of the Study:
- To investigate the presence and function of cardiac adenosine receptors in ventricular and atrial myocytes.
- To determine the specific roles of A1 and A2 adenosine receptor subtypes in regulating adenylate cyclase activity and contractility.
- To explore the differential expression of adenosine receptor subtypes in cardiac chambers.
Main Methods:
- Primary cultures of ventricular and atrial myocytes from embryonic chicks (14 days in ovo).
- Pharmacological blockade of A1-adenosine receptors using DPCPX or pertussis toxin treatment.
- Measurement of adenylate cyclase activity in response to various adenosine receptor agonists.
- Assessment of contractile amplitude in intact myocytes under stimulated conditions.
Main Results:
- In ventricular myocytes, blocking A1 receptors unmasked a stimulatory effect of adenosine agonists on adenylate cyclase activity, primarily mediated by A2 receptors.
- A2-selective agonists significantly increased adenylate cyclase activity and contractile amplitude in ventricular myocytes, effects absent in atrial myocytes.
- Both A1 and A2 adenosine receptor subtypes were found in ventricular myocytes, mediating opposing responses, while only A1 was detected in atrial myocytes.
Conclusions:
- A stimulatory adenosine receptor, likely A2, is present and functionally active in ventricular myocytes, directly linked to enhanced cardiac contractility.
- The absence of a functional A2 receptor in atrial myocytes suggests chamber-specific regulation of cardiac function by adenosine.
- The dual presence of opposing adenosine receptor subtypes in ventricles contributes to the complex inotropic effects of adenosine in the heart.