Enhanced chronotropic and inotropic responses of rat myocardium to cholinergic stimulus with aging

Assessing the effect of breed, age, and sex on muscarinic receptor distribution in atria and ventricles of turkeys

Atrial and ventricular myocardium from young (6-wk-old), young adult (3-6-mo-old), and aged (14-15-mo-old) meat-type (B.U.T. Big 6) and wild-type (Cröllwitzer) turkeys were used to study the influence of age and sex on cholinergic muscarinic receptors using [3H]-N-methyl-scopolamine (3H-NMS) binding studies. In both breeds, saturation experiments indicated the presence of regional-, sex-, and age-related differences in the density of cholinergic muscarinic receptors (Bmax), that is, a decrease or increase. Except for right atria, Bmax was decreased in both male and female B.U.T. Big 6 hearts with increasing age. Similarly, a negative correlation between Bmax and age could be seen in female and male atria of Cröllwitzer turkeys, while positive correlation could be seen in right and left ventricles of male, and only right ventricles of female Cröllwitzer turkeys. The affinity of the receptor (KD) was not affected by age, sex and breed. In all cardiac chamber tissues, the M2-subtype was shown to be predominant followed by the M3-subtype and to a lesser extent the M1-subtype. Aspects of this age-dependent remodeling of the heart differ between sexes, resulting in maladaptive changes in older turkeys with a high degree of frailty. These observations may help explain why males and females are susceptible to different cardiovascular diseases as they age and why frail older adults are most often affected by these diseases.

Age-associated abnormalities of intrinsic automaticity of sinoatrial nodal cells are linked to deficient cAMP-PKA-Ca(2+) signaling

A reduced sinoatrial node (SAN) functional reserve underlies the age-associated decline in heart rate acceleration in response to stress. SAN cell function involves an oscillatory coupled-clock system: the sarcoplasmic reticulum (SR), a Ca(2+) clock, and the electrogenic-sarcolemmal membrane clock. Ca(2+)-activated-calmodulin-adenylyl cyclase/CaMKII-cAMP/PKA-Ca(2+) signaling regulated by phosphodiesterase activity drives SAN cells automaticity. SR-generated local calcium releases (LCRs) activate Na(+)/Ca(2+) exchanger in the membrane clock, which initiates the action potential (AP). We hypothesize that SAN cell dysfunctions accumulate with age. We found a reduction in single SAN cell AP firing in aged (20-24 mo) vs. adult (3-4 mo) mice. The sensitivity of the SAN beating rate responses to both muscarinic and adrenergic receptor activation becomes decreased in advanced age. Additionally, age-associated coincident dysfunctions occur stemming from compromised clock functions, including a reduced SR Ca(2+) load and a reduced size, number, and duration of spontaneous LCRs. Moreover, the sensitivity of SAN beating rate to a cAMP stress induced by phosphodiesterase inhibitor is reduced, as are the LCR size, amplitude, and number in SAN cells from aged vs. adult mice. These functional changes coincide with decreased expression of crucial SR Ca(2+)-cycling proteins, including SR Ca(2+)-ATPase pump, ryanodine receptors, and Na(+)/Ca(2+) exchanger. Thus a deterioration in intrinsic Ca(2+) clock kinetics in aged SAN cells, due to deficits in intrinsic SR Ca(2+) cycling and its response to a cAMP-dependent pathway activation, is involved in the age-associated reduction in intrinsic resting AP firing rate, and in the reduction in the acceleration of heart rate during exercise.

Increased expression of Gi-coupled muscarinic acetylcholine receptor and Gi in atrium of elderly diabetic subjects

In an ongoing investigation of the effects of age on G protein-coupled receptor signaling in human atrial tissue, we have found that the density of atrial muscarinic acetylcholine receptor (mAChR) increases with age but reaches statistical significance only in patients with diabetes. Moreover, we find that in elderly subjects of similar ages, those with diabetes have 1.7-fold higher levels of Galpha(i2) and twofold higher levels of Gbeta(1). Diabetes does not affect other atrial G proteins, including Galpha(i3,) Galpha(s), Galpha(o), and Gbeta(2). These data represent the first demonstration of an increase in a G(i)-coupled receptor, Galpha(i2), and Gbeta(1), in atrium of patients with diabetes. These findings suggest a molecular explanation for the increased risk of cardiac disease in patients with diabetes, because increased signaling through G(i) has been shown to lead to the development of dilated cardiomyopathy.

Effects of aging on cardiovascular responses to parasympathetic withdrawal

OBJECTIVES

The study was done to determine whether the effects of parasympathetic withdrawal on heart rate, blood pressure (BP), and systolic and diastolic function are altered with normal aging.

BACKGROUND

Cardiac responses to beta-adrenergic sympathetic stimulation decline with aging as does the heart rate response to parasympathetic withdrawal, but the extent to which other responses to parasympathetic withdrawal decrease is less clear.

METHODS

Heart rate, BP, systolic function, and diastolic filling responses to parasympathetic withdrawal induced by atropine (0.02 mg/kg) were compared in 50 healthy subjects, 28 older (ages 65 to 80 years, mean 70 years; 18 females all on estrogen) and 22 young (age 18 to 32 years, mean 26 years; 12 females) subjects, using radionuclide angiography.

RESULTS

Parasympathetic withdrawal in the older group caused less of an increase in heart rate (+33 vs. +48 beats/min), cardiac index (+0.6 vs. +1.5 l/m(2)), systolic blood pressure (-1 vs. +7 mm Hg), and early diastolic filling rate (+1.7 vs. +2.4 end-diastolic volumes/s) (all p < or = 0.01). At similar declines in the diastolic filling period, end-diastolic volume index (EDVI) fell substantially more in the older group (-11.6 vs. -2.4 ml/m(2), p < 0.001). The only gender difference was in diastolic filling rate, which was similar in the young males and females, but significantly less in older males than in older females.

CONCLUSIONS

The responses to parasympathetic withdrawal as well as sympathetic stimulation decline with aging, and both contribute to the reduced cardiovascular responses to stress with advancing age.

Muscarinic receptors in the mammalian heart

In the mammalian heart, cardiac function is under the control of the sympathetic and parasympathetic nervous system. All regions of the mammalian heart are innervated by parasympathetic (vagal) nerves, although the supraventricular tissues are more densely innervated than the ventricles. Vagal activation causes stimulation of cardiac muscarinic acetylcholine receptors (M-ChR) that modulate pacemaker activity via I(f) and I(K.ACh), atrioventricular conduction, and directly (in atrium) or indirectly (in ventricles) force of contraction. However, the functional response elicited by M-ChR-activation depends on species, age, anatomic structure investigated, and M-ChR-agonist concentration used. Among the five M-ChR-subtypes M(2)-ChR is the predominant isoform present in the mammalian heart, while in the coronary circulation M(3)-ChR have been identified. In addition, evidence for a possible existence of an additional, not M(2)-ChR in the heart has been presented. M-ChR are subject to regulation by G-protein-coupled-receptor kinase. Alterations of cardiac M(2)-ChR in age and various kinds of disease are discussed.

Role of acetylcholine in electrical stimulation-induced arrhythmia in rat isolated atria

In this study, we used the spontaneously beating, isolated rat right atrium as an in vitro model to study arrhythmogenic effects of electrical stimulation. A tetrapolar platinum electrode was used for stimulation and recording of atrial electrical activity at 36.5 degrees C (spontaneous rate, 4.9+/-0.3 Hz). A flutter-like pattern of arrhythmia was reproducibly induced by application of stimulus trains (250 pulses, 66.7 Hz). Arrhythmia was characterized by regular and very short cycle length (40-70 ms), each episode lasting from 3 s to >5 min. In control conditions, application of one to five pulse trains was sufficient to induce arrhythmia. However, atropine (but not propranolol) completely blocked arrhythmia induction (10-15 consecutive trains were ineffective). The ability of electrical stimulation to evoke arrhythmia was restored after atropine washout. A milder stimulation protocol (30 pulses, 50 Hz), which was unable to evoke arrhythmia in control conditions, was fully effective in the presence of 1 microM acetylcholine (ACh). Furthermore, a similar flutter-like pattern could be induced in isolated left atria in the presence of ACh. Our results point out an arrhythmogenic effect of neurally released ACh in the isolated right atrium on atrial electrical stimulation.

Diurnal rhythms in norepinephrine and acetylcholine synthesis of sympathetic ganglia, heart and adrenals of aging rats: effect of melatonin

The effect of aging and melatonin on 24-h rhythms in tyrosine hydroxylase activity and 3H - choline conversion into 3H - acetylcholine were examined in cervical, stellate, coeliac-mesenteric and hypogastric ganglia, and in the adrenal medulla and heart of rats. Young (50 days old) and old (18 months old) rats received evening injections of 10 or 100 microg of melatonin or its vehicle for 17 days. In superior cervical, stellate and coeliac-superior mesenteric ganglia, as well as in the adrenal medulla, norepinephrine and acetylcholine synthesis attained maximal values at night (c.a. 2030-0100 h). In the hypogastric ganglion, maximal tyrosine hydroxylase activity occurred at night at both studied ages. Two maxima in acetylcholine synthesis were detected in hypogastric ganglion of young rats (c.a. 1300 h and 0100 h, respectively) while in old rats a single maximum was observed at noon. Cardiac tyrosine hydroxylase activity peaked at early night (c.a. 2200-2300 h) while cardiac acetylcholine synthesis peaked at the afternoon (c.a. 1700-1900 h). Old rats exhibited a significant decrease of rhythm amplitude and increase of mean values in tyrosine hydroxylase activity in autonomic ganglia and adrenal medulla, and abolition of tyrosine hydroxylase rhythm in the heart. Twenty-four hour rhythmicity in acetylcholine synthesis was impaired or abolished in aged rats. Treatment of old rats with 10 or 100 microg melatonin generally augmented amplitude of rhythms and reinduced the nocturnal peak of acetylcholine synthesis in the hypogastric ganglion. Only the high melatonin dose significantly augmented rhythm amplitude of tyrosine hydroxylase activity (superior cervical and coeliac-superior mesenteric ganglia) and acetylcholine synthesis (superior cervical, stellate and coeliac-superior mesenteric ganglia) in young rats. The results indicate that the activity of the central oscillator, driven to the organs in part via the autonomic nervous system, deteriorates significantly with aging and that melatonin may restore partially such a deterioration.

Beta-adrenergic and muscarinic receptor mRNA accumulation in the sinoatrial node area of adult and senescent rat hearts

The sinoatrial (SA) node is the cardiac pacemaker and changes in its adrenergic-muscarinic phenotype have been postulated as a determinant of age-associated modifications in heart rate variability. To address this question, right atria were microdissected, the SA node area was identified by acetylcholinesterase staining, and, using a RT-PCR method, the accumulation of mRNA molecules encoding beta1- and beta2-adrenergic (beta1- and beta2-AR) and muscarinic (M2-R) receptor was quantified to define the proportion between beta-AR and M2-R mRNAs within the sinoatrial area of adult (3 months) and senescent (24 months) individual rat hearts. In adult hearts, the highest M2-R/beta-AR mRNA ratio was observed within the sinoatrial area compared with adjacent atrial myocardium, while in the senescent hearts, no difference was observed between sinoatrial and adjacent areas. This change was specific of the sinoatrial area since adult and senescent whole atrial or ventricular myocardium did not differ in their M2-R/beta-AR mRNA ratio, and was associated with a fragmentation of acetylcholinesterase staining of the senescent SA node. Quantitative changes in the expression of genes encoding proteins involved in heart rate regulation specifically affect the sinoatrial area of the senescent heart.

Age-related changes in electrophysiological responses to muscarinic receptor stimulation in rat myocardium

Recent studies have demonstrated that the negative chronotropic and inotropic responses of the heart to cholinergic muscarinic receptor stimulation are strikingly enhanced with aging in the rat model. The present study investigated the electrophysiological basis of this phenomenon by determining the effects of a muscarinic receptor agonist, carbachol, on transmembrane action potential parameters in right atrial tissue and right ventricular free wall preparations from adult (6-8 months old) and aged (26-28 months old) Fischer 344 rats. In addition, the effect of carbachol on atrioventricular conduction time (AVT) was determined in isolated perfused beating hearts. The results showed the following. The baseline maximum diastolic potential (MDP: adult, -76.4 +/- 1.8 mV; aged, -66.8 +/- 1.5 mV; p < 0.05; n = 5) but not the action potential duration measured at 95% repolarization (APD95: adult, 40.0 +/- 5.0 ms; aged, 47.4 +/- 6.7 ms; n = 5) differed significantly in aged compared with adult atrium. No significant age-related difference was evident in baseline MDP measured in ventricular epicardium (adult, -69.8 +/- 0.5 mV; aged, -69.0 +/- 1.1 mV; n = 6) or endocardium (adult, -72.5 +/- 1.4 mV; aged, -73.0 +/- 1.2 mV; n =6). The baseline action potential duration measured at 50% repolarization (APD50) differed significantly with age in ventricular endocardium (adult, 11.6 +/- 2.2 ms; aged, 23.0 +/- 4.6 ms; p < 0.05; n =6) but not in epicardium (APD50: adult, 8.1 +/- 0.4 ms; aged, 13.0 +/- 2.3 ms; n = 6). Superfusion with carbachol (0.1 nM - 10 mu M) resulted in concentration-dependent hyperpolarization of MDP in atrium; the magnitude of hyperpolarization differed significantly with age (2.5-fold higher in the aged; p < 0.05; n = 5). Carbachol caused concentration-dependent shortening of APD50; this effect differed significantly with age in the ventricle (2-fold greater in the aged; p < 0.05; n = 6) but not in the atrium. Carbachol prolonged the AVT in atrial-paced (240 beats/min) hearts; the magnitude of carbachol-induced increase in AVT did not differ significantly with age. These results are consistent with the possibility that in the aging heart, greater hyperpolarization at the level of the right atrium (likely involving pacemaker cells) and greater shortening of APD50 at the level of ventricular myocytes may contribute to the enhanced cholinergic-triggered bradycardia and negative inotropic response, respectively.