Depressed pacemaker activity of sinoatrial node myocytes contributes to the age-dependent decline in maximum heart rate

Abstract

An inexorable decline in maximum heart rate (mHR) progressively limits human aerobic capacity with advancing age. This decrease in mHR results from an age-dependent reduction in "intrinsic heart rate" (iHR), which is measured during autonomic blockade. The reduced iHR indicates, by definition, that pacemaker function of the sinoatrial node is compromised during aging. However, little is known about the properties of pacemaker myocytes in the aged sinoatrial node. Here, we show that depressed excitability of individual sinoatrial node myocytes (SAMs) contributes to reductions in heart rate with advancing age. We found that agedependent declines in mHR and iHR in ECG recordings from mice were paralleled by declines in spontaneous action potential (AP) firing rates (FRs) in patch-clamp recordings from acutely isolated SAMs. The slower FR of aged SAMs resulted from changes in the AP waveform that were limited to hyperpolarization of the maximum diastolic potential and slowing of the early part of the diastolic depolarization. These AP waveform changes were associated with cellular hypertrophy, reduced current densities for L-and T-type Ca 2+ currents and the "funny current" (I f ), and a hyperpolarizing shift in the voltage dependence of I f . The agedependent reduction in sinoatrial node function was not associated with changes in β-adrenergic responsiveness, which was preserved during aging for heart rate, SAM FR, L-and T-type Ca 2+ currents, and I f . Our results indicate that depressed excitability of individual SAMs due to altered ion channel activity contributes to the decline in mHR, and thus aerobic capacity, during normal aging. O ne of the most insidious aspects of growing older is an inevitable decline in maximum heart rate (mHR), which limits maximum aerobic capacity with advancing age (1-3). The decline in mHR proceeds at approximately the same rate for all individuals, without regard for lifestyle or physical fitness (4-8). For many otherwise healthy elderly people, it is the factor that ultimately restricts the ability to live independently (9, 10).The decrease in mHR with age results primarily from a parallel age-dependent decline in "intrinsic heart rate" (iHR) (11-13), which is measured during autonomic blockade, and thus reflects the spontaneous pacemaker activity of the sinoatrial node of the heart. Although it is known that the intact sinoatrial node from aged animals contracts more slowly (14, 15) and contains fewer pacemaker myocytes (16), little is known about the functional properties of individual myocytes from the sinoatrial node of the aged heart.Sinoatrial myocytes (SAMs) are highly specialized cells that serve a primarily electrical function as cardiac pacemakers via their production of spontaneous action potentials (APs). Sinoatrial APs are characterized by a spontaneous depolarization during diastole that drives the membrane potential to threshold, thereby triggering the subsequent AP. This "diastolic depolarization" (DD) phase of the sinoatrial AP results from the coordinated activit...

Culture and adenoviral infection of sinoatrial node myocytes from adult mice

“…SAMs were harvested from male C57BL/6J mice of 6 -12 wk of age (Jackson Laboratories, Bar Harbor, ME) as we have previously described (16,18,38,14). Animals were anesthetized by inhalation of isoflurane and euthanized by cervical dislocation.…”

“…SAMs were isolated from the sinoatrial node, which was defined as the region bordered by the crista terminalis, the interatrial septum, and the superior and inferior vena cavae as in previous studies (16,18,38,14,25,33). Individual cells were initially identified visually by the presence of spontaneous contractions and by their characteristic morphology, small size, and lack of striations as we have previously described (14,16,18,38). In electrophysiological recordings, SAMs were additionally defined as cells that fired spontaneous action potentials with characteristic AP waveform parameters including a diastolic depolarization phase and a slow upstroke (14) and by the presence of If of Ͼ100 pA in response to a 1-s voltage step to Ϫ120 mV in voltage clamp experiments (see RESULTS).…”

“…Individual cells were initially identified visually by the presence of spontaneous contractions and by their characteristic morphology, small size, and lack of striations as we have previously described (14,16,18,38). In electrophysiological recordings, SAMs were additionally defined as cells that fired spontaneous action potentials with characteristic AP waveform parameters including a diastolic depolarization phase and a slow upstroke (14) and by the presence of If of Ͼ100 pA in response to a 1-s voltage step to Ϫ120 mV in voltage clamp experiments (see RESULTS). All of the acutely isolated and cultured SAMs that were assayed were also found to express HCN4 channel protein in immunocytochemical experiments (see Fig.…”

“…Data were acquired at 5 kHz and low-pass filtered at 1 kHz using an Axopatch 1D or 200B amplifier, Digidata 1322a or 1440a A/D converter, and ClampEx software (Molecular Devices). Spontaneous AP firing rate was measured in the presence of 1 nM isoproterenol (ISO; EMD Millipore/Calbiochem, Billerica, MA) in the bathing Tyrode's solution, which was used to promote stable AP firing in both acutely isolated and cultured SAMs as previously described (6,14,16,18,38).…”

Fibrosis, Electrics and Genetics

“…An important observation is that single isolated SAN cells from mice have a slower spontaneous firing rate with aging, and clearly this cannot be explained by fibrosis. 31 In humans, detailed electrophysiological study of patients with idiopathic SND have revealed widespread conduction delay that is especially pronounced along the CT, caudal shift of the leading pacemaker and a change from a multicentric (in controls) to a unicentric site of first activation (Figure 3). 25 The molecular correlate of these changes is, at least in part, ion channel remodeling, which has been extensively investigated in aging animal models of SND.…”