Abstract-Local, rhythmic, subsarcolemmal Ca 2ϩ releases (LCRs) from the sarcoplasmic reticulum (SR) during diastolic depolarization in sinoatrial nodal cells (SANC) occur even in the basal state and activate an inward Na ϩ -Ca 2ϩ exchanger current that affects spontaneous beating. Why SANC can generate spontaneous LCRs under basal conditions, whereas ventricular cells cannot, has not previously been explained. Here we show that a high basal cAMP level of isolated rabbit SANC and its attendant increase in protein kinase A (PKA)-dependent phosphorylation are obligatory for the occurrence of spontaneous, basal LCRs and for spontaneous beating. Gradations in basal PKA activity, indexed by gradations in phospholamban phosphorylation effected by a specific PKA inhibitory peptide were highly correlated with concomitant gradations in LCR spatiotemporal synchronization and phase, as well as beating rate. Higher levels of basal PKA inhibition abolish LCRs and spontaneous beating ceases. Stimulation of ␤-adrenergic receptors extends the range of PKA-dependent control of LCRs and beating rate beyond that in the basal state. The link between SR Ca 2ϩ cycling and beating rate is also present in vivo, as the regulation of beating rate by local ␤-adrenergic receptor stimulation of the sinoatrial node in intact dogs is markedly blunted when SR Ca 2ϩ cycling is disrupted by ryanodine. Thus, PKA-dependent phosphorylation of proteins that regulate cell Ca 2ϩ balance and spontaneous SR Ca 2ϩ cycling, ie, phospholamban and L-type Ca 2ϩ channels (and likely others not measured in this study), controls the phase and size of LCRs and the resultant Na ϩ -Ca 2ϩ exchanger current and is crucial for both basal and reserve cardiac pacemaker function. R ecent studies have demonstrated that in sinoatrial (SA) nodal cells (SANC) generate local, rhythmic, subsarcolemmal Ca 2ϩ releases (LCRs) under basal conditions, ie, even in the absence of experimental Ca 2ϩ loading or stimulation of ␤-adrenergic receptors (␤-ARs). [1][2][3] In rabbit SANC, spontaneous, rhythmic LCRs occur during the late diastolic depolarization and activate Na ϩ -Ca 2ϩ exchanger (NCX) to generate an inward current that accelerates the depolarization rate, and, thus, LCRs are involved in control of spontaneous beating rate of SANC. 1 The mechanisms that permit SANC, but not ventricular myocytes, to generate rhythmic LCRs under basal conditions, however, have not been delineated.Spontaneous SR Ca 2ϩ release is facilitated by factors that increase the rate at which the SR can pump Ca 2ϩ , foremost among which are elevated cell Ca 2ϩ or elevated cAMP and its attendant protein kinase A (PKA)-dependent protein phosphorylation that results from intense ␤-AR stimulation. Whereas the cytosolic Ca 2ϩ concentration does not appreciably differ in rabbit ventricular cells and SANC, 2,4 the cAMP level of the intact SA node is high, 5 and it has been suspected that the basal cAMP level within SANC is elevated. 6,7 The SA node, however, is highly innervated, and neither the basal cAMP le...

Padilla J, Young CN, Simmons GH, Deo SH, Newcomer SC, Sullivan JP, Laughlin MH, Fadel PJ. Increased muscle sympathetic nerve activity acutely alters conduit artery shear rate patterns. Am J Physiol Heart Circ Physiol 298: H1128 -H1135, 2010. First published February 12, 2010 doi:10.1152/ajpheart.01133.2009.-Escalating evidence indicates that disturbed flow patterns, characterized by the presence of retrograde and oscillatory shear stress, induce a proatherogenic endothelial cell phenotype; however, the mechanisms underlying oscillatory shear profiles in peripheral conduit arteries are not fully understood. We tested the hypothesis that acute elevations in muscle sympathetic nerve activity (MSNA) are accompanied by increases in conduit artery retrograde and oscillatory shear. Fourteen healthy men (25 Ϯ 1 yr) performed three sympathoexcitatory maneuvers: graded lower body negative pressure (LBNP) from 0 to Ϫ40 Torr, cold pressor test (CPT), and 35% maximal voluntary contraction handgrip followed by postexercise ischemia (PEI). MSNA (microneurography; peroneal nerve), arterial blood pressure (finger photoplethysmography), and brachial artery velocity and diameter (duplex Doppler ultrasound) in the contralateral arm were recorded continuously. All maneuvers elicited significant increases in MSNA total activity from baseline (P Ͻ 0.05). Retrograde shear (Ϫ3.96 Ϯ 1.2 baseline vs. Ϫ8.15 Ϯ 1.8 s Ϫ1 , Ϫ40 LBNP, P Ͻ 0.05) and oscillatory shear index (0.09 Ϯ 0.02 baseline vs. 0.20 Ϯ 0.02 arbitrary units, Ϫ40 LBNP, P Ͻ 0.05) were progressively augmented during graded LBNP. In contrast, during CPT and PEI, in which MSNA and blood pressure were concomitantly increased (P Ͻ 0.05), minimal or no changes in retrograde and oscillatory shear were noted. These data suggest that acute elevations in MSNA are associated with an increase in conduit artery retrograde and oscillatory shear, an effect that may be influenced by concurrent increases in arterial blood pressure. Future studies should examine the complex interaction between MSNA, arterial blood pressure, and other potential modulatory factors of shear rate patterns. sympathetic tone; blood pressure; blood flow; endothelium; atherosclerosis ATHEROSCLEROSIS IS A POTENTIALLY life-threatening disease of large and medium-size arteries that is strongly associated with systemic risk factors such as physical inactivity, obesity, hypercholesterolemia, hypertension, smoking, and diabetes (30). However, the predictable distribution of atherosclerosis indicates that localized hemodynamic environments may also influence the atherosclerotic disease process (31). Indeed, postmortem (36) and in vivo imaging studies (10) demonstrate that atherosclerotic lesions are preferentially located in regions distinguished by oscillatory (bidirectional blood flow) and low mean shear stress, whereas areas exposed to unidirectional and moderate shear are protected. A causal link between disturbed flow patterns and a proatherogenic endothelial cell phenotype has been extensively confirmed by in vitro studi...

Recent animal studies indicate that insulin increases arterial baroreflex control of lumbar sympathetic nerve activity; however, the extent to which these findings can be extrapolated to humans is unknown. To begin to address this, muscle sympathetic nerve activity (MSNA) and arterial blood pressure were measured in 19 healthy subjects (27 ± 1 years) before, and for 120 min following, two common methodologies used to evoke sustained increases in plasma insulin: a mixed meal and a hyperinsulinaemic euglycaemic clamp. Weighted linear regression analysis between MSNA and diastolic blood pressure was used to determine the gain (i.e. sensitivity) of arterial baroreflex control of MSNA. Plasma insulin was significantly elevated within 30 min following meal intake ( 34 ± 6 uIU ml −1 ; P < 0.05) and remained above baseline for up to 120 min. Similarly, after meal intake, arterial baroreflex-MSNA gain for burst incidence and total MSNA was increased and remained elevated for the duration of the protocol (e.g. burst incidence gain: −3.29 ± 0.54 baseline vs. −5.64 ± 0.67 bursts (100 heart beats) −1 mmHg −1 at 120 min; P < 0.05). During the hyperinsulinaemic euglycaemic clamp, in which insulin was elevated to postprandial concentrations ( 42 ± 6 μIU ml −1 ; P < 0.05), while glucose was maintained constant, arterial baroreflex-MSNA gain was similarly enhanced (e.g. burst incidence gain: −2.44 ± 0.29 baseline vs. −4.74 ± 0.71 bursts (100 heart beats)at 120 min; P < 0.05). Importantly, during time control experiments, with sustained fasting insulin concentrations, the arterial baroreflex-MSNA gain remained unchanged. These findings demonstrate, for the first time in healthy humans, that increases in plasma insulin enhance the gain of arterial baroreflex control of MSNA.

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