Myocardial cholinergic signaling changes with age

The detection of the non-M2 muscarinic receptor subtype in the rat heart atria and ventricles

Mammal heart tissue has long been assumed to be the exclusive domain of the M(2) subtype of muscarinic receptor, but data supporting the presence of other subtypes also exist. We have tested the hypothesis that muscarinic receptors other than the M(2) subtype are present in the heart as minor populations. We used several approaches: a set of competition binding experiments with pirenzepine, AFDX-116, 4-DAMP, PD 102807, p-F-HHSiD, AQ-RA 741, DAU 5884, methoctramine and tripinamide, blockage of M(1) muscarinic receptors using MT7 toxin, subtype-specific immunoprecipitation experiments and determination of phospholipase C activity. We also attempted to block M(1)-M(4) receptors using co-treatment with MT7 and AQ-RA 741. Our results show that only the M(2) subtype is present in the atria. In the ventricles, however, we were able to determine that 20% (on average) of the muscarinic receptors were subtypes other than M(2), with the majority of these belonging to the M(1) subtype. We were also able to detect a marginal fraction (6 +/- 2%) of receptors that, based on other findings, belong mainly to the M(5) muscarinic receptors. Co-treatment with MT7 and AQ-RA 741 was not a suitable tool for blocking of M(1)-M(4) receptors and can not therefore be used as a method for M(5) muscarinic receptor detection in substitution to crude venom. These results provide further evidence of the expression of the M(1) muscarinic receptor subtype in the rat heart and also show that the heart contains at least one other, albeit minor, muscarinic receptor population, which most likely belongs to the M(5) muscarinic receptors but not to that of the M(3) receptors.

Effects of gestational age and cortisol treatment on ovine fetal heart function in a novel biventricular Langendorff preparation

Structural and functional maturation of a number of fetal organs and physiological systems occurs in the immediate period prior to term, in association with the prepartum increase in fetal plasma cortisol concentration. At present, little is known about how myocardial sensitivity to adrenergic and muscarinic cholinergic stimulation changes as the fetus approaches term, nor the role of the prepartum increase in plasma cortisol concentration in mediating these changes. This study used a novel Langendorff, biventricular, ovine fetal heart preparation to investigate the effects of advancing gestation and cortisol treatment on myocardial sensitivity to adrenergic (isoprenaline) and muscarinic cholinergic (carbachol) stimulation. It was hypothesized that cortisol infusion would fully mimic developmental changes in myocardial responsiveness to adrenergic and cholinergic stimulation. Sixteen Welsh Mountain sheep fetuses were surgically prepared under general anaesthesia with vascular catheters. At 125 +/- 1 days gestational age (dGA; term, 145 dGA) fetuses were infused with saline vehicle (n= 7; Premature Control) or with cortisol (n= 4; 2-3 mg kg(-1) d(-1)i.v.; Premature Cortisol) for 5 days. The Term Control group (n= 5) comprised fetuses that were surgically prepared at 130 dGA and infused with vehicle for 5 days prior to delivery (n= 2), or that received no surgery (n= 3). Under terminal anaesthesia, Premature Control and Premature Cortisol fetuses were delivered at 130 dGA and Term Control fetuses between 135 and 143 dGA. Following exsanguination under anaesthesia, fetal hearts were mounted in the Langendorff preparation, allowing measurement of left ventricular (LV) developed pressure and right ventricular (RV) developed pressure, heart rate (HR), coronary perfusion pressure and perfusate distribution to the myocardium. Cortisol infusion elevated fetal plasma cortisol concentrations to values similar to those measured close to term (45.0 +/- 7.1 ng ml(-1)). Advancing gestational age, but not cortisol treatment, enhanced fetal LV developed pressure, RV developed pressure and HR responses to carbachol (P < 0.05). Advancing gestational age, but not cortisol treatment, suppressed fetal LV developed pressure, RV developed pressure and HR responses to isoprenaline (P < 0.05). Maximum doses of either carbachol or isoprenaline had no effect on coronary perfusate distribution. Changes in myocardial responsiveness to adrenergic and muscarinic cholinergic stimulation with advancing gestation provide mechanisms that contribute to the maturation of the cardiovascular system as the ovine fetus approaches term. These changes in myocardial responsiveness are not solely dependent on preparturient elevations in fetal plasma cortisol concentration.

Cavopulmonary anastomosis induces pulmonary expression of the angiotensin II receptor family

BACKGROUND

Cavopulmonary anastomosis is used for palliation of cyanotic cardiac lesions. Postoperative development of pulmonary arteriovenous malformations can be significant in 10% to 25% of patients. To study the basis for formation of arteriovenous malformations, we developed an ovine model that reliably induces their development 8 weeks after cavopulmonary anastomosis. Previously, we found that cavopulmonary anastomosis inhibits the expression of pulmonary angiotensin-converting enzyme and suppresses angiotensin II production.

OBJECTIVE

This study examines the role of the angiotensin II receptors, type 1 and type 2, in this setting of pulmonary vascular remodeling.

METHODS

Lambs, aged 40 to 50 days, underwent cavopulmonary anastomosis. In age-matched control animals, a sham operation was performed. Messenger RNA and protein expression in lung specimens was measured at successive time points after cavopulmonary anastomosis or sham operations (n = 3 at each time point).

RESULTS

Angiotensin type 1 mRNA was maximally upregulated 2-fold at 5 weeks after cavopulmonary anastomosis (P =.006). Expression of angiotensin type 1 protein was increased at least 2-fold at 2, 5, and 15 weeks after cavopulmonary anastomosis (P =.005). Cavopulmonary anastomosis also increased angiotensin type 2 mRNA and protein expression at least 2-fold at 2 and 5 weeks (P =.02) after surgical intervention. At 15 weeks, expression of angiotensin type 2 mRNA and protein was unchanged from that seen in control animals. Immunolocalization in pulmonary tissue sections 2 weeks after cavopulmonary anastomosis revealed markedly enhanced staining of angiotensin II receptor type 1 in vascular smooth muscle and angiotensin II receptor type 2 in the endothelium of pulmonary arteries.

CONCLUSIONS

Rapid elevation in the expression of the type 1 and 2 angiotensin II receptors in the affected pulmonary vasculature after cavopulmonary anastomosis suggests their involvement in the pathologic vascular remodeling that occurs after cavopulmonary anastomosis.

Acetylcholine chloride and renal hemodynamics during postnatal maturation in conscious lambs

To test the hypothesis that acetylcholine-induced relaxation of the renal artery decreases with postnatal age, we measured parameters of renal hemodynamics before and for 35 s after aortic suprarenal injection of acetylcholine in conscious, chronically instrumented lambs aged approximately 1 wk (n = 5) and approximately 6 wk (n = 5). Acetylcholine was administered in one of five doses ranging from 0 to 10 mg/kg body wt; doses were administered randomly, in the same volume. There were significant age- and dose-dependent changes in renal vascular resistance after acetylcholine administration, such that the response was greater in 1-wk-old lambs. After the highest dose tested, renal vascular resistance decreased by 13.6 +/- 7.3 (SD) mmHg. ml(-1). min. g kidney wt in 1-wk-old lambs and by 9.1 +/- 3.2 mmHg. ml(-1). min. g kidney wt in 6-wk-old lambs at 35 s. We also observed a transient renal vasoconstriction before the renal vasodilatation in 6-wk-old lambs but not in 1-wk-old animals. These data provide the first age- and dose-dependent effects of exogenous administration of acetylcholine on renal hemodynamics during maturation in conscious animals.

Characterisation of subtypes of the P2X and P2Y families of ATP receptors in the foetal human heart

ATP exerts a variety of actions within the myocardium, including the regulation of coronary vascular tone and modulation of the autonomic control of the heart. In order to characterise the ATP receptor subtypes involved in these effects, degenerate oligonucleotides were used to clone receptors of both P2X and P2Y families from the human foetal heart. About 1 ng of "Quick-Clone cDNA" from foetal human heart was subjected to amplification with two pairs of degenerate oligonucleotides designed to amplify subtypes of the P2X and P2Y receptor families by means of PCR reactions. The sequence analysis of 34 and 29 clones of the P2X and P2Y receptor families, respectively, demonstrated that P2X1, P2X3 and P2X4 subtypes are present in the human foetal heart together with P2Y6, P2Y2 and P2Y4 receptors. P2X1 and P2Y4 receptor subtypes were here characterised for the first time in the human foetal heart. The present study provides the first molecular characterisation of ATP receptors in the foetal human heart. The results show that many P2 receptor subtypes are expressed in the foetal human heart, perhaps contributing to developmental processes as well as to the activity of the foetal heart.