Central activation of cardiac vagal nerve by α2-adrenergic stimulation is impaired in streptozotocin-induced type 1 diabetic rats

Acute effects of empagliflozin on open-loop baroreflex function and urine output in streptozotocin-induced type 1 diabetic rats

Although sympathetic suppression is considered one of the mechanisms for cardioprotection afforded by sodium-glucose cotransporter 2 (SGLT2) inhibitors, whether SGLT2 inhibition acutely modifies sympathetic arterial pressure (AP) regulation remains unclear. We examined the acute effect of an SGLT2 inhibitor, empagliflozin (10 mg/kg), on open-loop baroreflex static characteristics in streptozotocin (STZ)-induced type 1 diabetic and control (CNT) rats (n = 9 each). Empagliflozin significantly increased urine flow [CNT: 25.5 (21.7-31.2) vs. 55.9 (51.0-64.5), STZ: 83.4 (53.7-91.7) vs. 121.2 (57.0-136.0) μL·min-1·kg-1, median (1st-3rd quartiles), P < 0.001 for empagliflozin and STZ]. Empagliflozin decreased the minimum sympathetic nerve activity (SNA) [CNT: 15.7 (6.8-18.4) vs. 10.5 (2.9-19.0), STZ: 36.9 (25.7-54.9) vs. 32.8 (15.1-37.5) %, P = 0.021 for empagliflozin and P = 0.003 for STZ], but did not significantly affect the peripheral arc characteristics assessed by the SNA-AP relationship. Despite the significant increase in urine flow and changes in several baroreflex parameters, empagliflozin preserved the overall sympathetic AP regulation in STZ-induced diabetic rats. The lack of a significant change in the peripheral arc may minimize reflex sympathetic activation, thereby enhancing a cardioprotective benefit of empagliflozin.

Contribution of afferent pathway to vagal nerve stimulation-induced myocardial interstitial acetylcholine release in rats

Vagal nerve stimulation (VNS) has been explored as a potential therapy for chronic heart failure. The contribution of the afferent pathway to myocardial interstitial acetylcholine (ACh) release during VNS has yet to be clarified. In seven anesthetized Wistar-Kyoto rats, we implanted microdialysis probes in the left ventricular free wall and measured the myocardial interstitial ACh release during right VNS with the following combinations of stimulation frequency (F in Hz) and voltage readout (V in volts): F0V0 (no stimulation), F5V3, F20V3, F5V10, and F20V10. F5V3 did not affect the ACh level. F20V3, F5V10, and F20V10 increased the ACh level to 2.83 ± 0.47 (P < 0.01), 4.31 ± 1.09 (P < 0.001), and 4.33 ± 0.82 (P < 0.001) nM, respectively, compared with F0V0 (1.76 ± 0.22 nM). After right vagal afferent transection (rVAX), F20V3 and F20V10 increased the ACh level to 2.90 ± 0.53 (P < 0.001) and 3.48 ± 0.63 (P < 0.001) nM, respectively, compared with F0V0 (1.61 ± 0.19 nM), but F5V10 did not (2.11 ± 0.24 nM). The ratio of the ACh levels after rVAX relative to before was significantly <100% in F5V10 (59.4 ± 8.7%) but not in F20V3 (102.0 ± 8.7%). These results suggest that high-frequency and low-voltage stimulation (F20V3) evoked the ACh release mainly via direct activation of the vagal efferent pathway. By contrast, low-frequency and high-voltage stimulation (F5V10) evoked the ACh release in a manner dependent on the vagal afferent pathway.

Threshold and saturation pressures of baroreflex-mediated myocardial interstitial acetylcholine release in rats

Cardiac microdialysis allows the assessment of cardiac efferent vagal nerve activity from myocardial interstitial acetylcholine (ACh) levels with minimal influence on the neural control of the heart; however, a total picture of the baroreflex-mediated myocardial interstitial ACh release including the threshold and saturation pressures has yet to be quantified. In eight anesthetized Wistar-Kyoto rats, we implanted microdialysis probes in the left ventricular free wall and measured the myocardial interstitial ACh release simultaneously with efferent sympathetic nerve activity (SNA) during a carotid sinus baroreceptor pressure input between 60 and 180 mm Hg. The baroreflex-mediated ACh release approximated a positive sigmoid curve, and its threshold and saturation pressures were not significantly different from those of an inverse sigmoid curve associated with the baroreflex-mediated SNA response (threshold: 94.3 ± 8.6 vs. 99.3 ± 6.0 mm Hg; saturation: 150.0 ± 10.3 vs. 158.8 ± 5.8 mm Hg). The sympathetic and vagal systems have certain levels of activities across most of the normal pressure range.

Accentuated antagonism of vagal heart rate control and less potent prejunctional inhibition of vagal acetylcholine release during sympathetic nerve stimulation in the rat

Complex interactions are known to occur between the sympathetic and parasympathetic controls of the heart. Although sympathetic nerve stimulation (SNS) usually augments the heart rate (HR) response to vagal nerve stimulation (VNS), exogenously administered norepinephrine (NE) can attenuate the HR response as well as the myocardial interstitial acetylcholine (ACh) release during VNS. To provide a basis for an integrative knowledge about the opposing adrenergic effects on the vagal control of the heart, we examined whether SNS significantly attenuates VNS-induced myocardial interstitial ACh release in the in vivo beating heart. In nine anesthetized rats, changes in HR and myocardial interstitial ACh release in response to 5- and 20-Hz VNS were examined in both the absence and presence of a 5-Hz background SNS. The SNS significantly enhanced the VNS-induced HR reduction during 20-Hz VNS (-101.2 ± 33.1 vs. -163.0 ± 34.9 beats/min, P < 0.001, a 60% augmentation). By contrast, the SNS significantly attenuated the ACh release during 20-Hz VNS (4.30 ± 0.72 vs. 3.80 ± 0.75 nM, P < 0.01, a 12% attenuation). In conclusion, SNS exerted only a moderate inhibitory effect on the VNS-induced myocardial interstitial ACh release in the in vivo beating heart.