Remote Limb Ischemic Preconditioning Attenuates Cerebrovascular Depression During Sinusoidal Galvanic Vestibular Stimulation via α1-Adrenoceptor-Protein Kinase Cε-Endothelial NO Synthase Pathway in Rats.

BACKGROUND: Vasovagal syncope (VVS) is characterized by hypotension and bradycardia followed by lowering of cerebral blood flow. Remote limb ischemic preconditioning (RIPC) is well documented to provide cardio- and neuroprotection as well as to improve cerebral blood flow. We hypothesized that RIPC will provide protection against VVS-induced hypotension, bradycardia, and cerebral hypoperfusion. Second, because endothelial nitric oxide synthase has been reported as a mediator of cerebral blood flow control, we hypothesized that the mechanism by which RIPC primes the vasculature against VVS is via the α1-adrenoceptor-protein kinase Cε-endothelial nitric oxide synthase pathway. METHODS AND RESULTS: We utilized sinusoidal galvanic vestibular stimulation in rats as a model of VVS. RIPC attenuated the lowerings of mean arterial pressure, heart rate, and cerebral blood flow caused by sinusoidal galvanic vestibular stimulation, as well as improving behavior during, and recovery after, stimulation. RIPC induced elevated serum norepinephrine, increased expression of brain α1-adrenoceptors, and reduced brain expression of norepinephrine transporter 1. Antagonizing adrenoceptors and norepinephrine transporter 1 prevented RIPC protection of cerebral perfusion during sinusoidal galvanic vestibular stimulation. CONCLUSIONS: Taken together, this study indicates that RIPC may be a potential therapy that can prevent VVS pathophysiology, decrease syncopal episodes, and reduce the injuries associated with syncopal falls. Furthermore, the α1-adrenoceptor-protein kinase Cε-endothelial nitric oxide synthase pathway may be a therapeutic target for regulating changes in cerebral blood flow.

A selective Akt inhibitor produces hypotension and bradycardia in conscious rats due to inhibition of the autonomic nervous system.

Akt is a serine-threonine kinase that is amplified in a variety of human cancers, and as with other anticancer agents, some Akt inhibitors have produced functional cardiovascular effects such as marked hypotension that may limit their clinical benefit. Although identified in preclinical studies, the mechanism(s) responsible for these effects are often not fully characterized; potential targets include Akt signaling disruption in cardiac tissue, vascular smooth muscle, and/or autonomic system signaling. A selective Akt inhibitor was found to produce a rapid and marked hypotension and bradycardia in conscious rats. Isolated right atrial tissue and isolated thoracic aortic rings were used to examine direct effects of Akt inhibition on cardiac and vascular tissues, respectively. In addition, rats surgically prepared with telemetry units for monitoring blood pressure and heart rate were used to investigate potential effects on the autonomic nervous system (ANS). Whereas this Akt inhibitor did not produce any significant effect on atrial tissue, it did cause vasorelaxation of aortic rings. More significantly, in conscious rats, the Akt inhibitor inhibited the neural pressor response to the known nicotinic acetylcholine receptor (nAchR) agonist dimethylphenylpiperazinium (DMPP). In fact, the response observed was comparable to the response observed with the known ganglionic blocker hexamethonium. Thus, the hypotension and bradycardia produced by the Akt inhibitor is primarily due to blockade of nAchRs in autonomic ganglia. This finding highlights the importance of evaluating the ANS for cardiovascular effects associated with new chemical entities as well as suggesting a novel direct effect of an Akt inhibitor on nAchRs.

Neuronal nitric oxide synthase gene transfer promotes cardiac vagal gain of function.

Nitric oxide (NO) generated from neuronal nitric oxide synthase (NOS-1) in intrinsic cardiac ganglia has been implicated in parasympathetic-induced bradycardia. We provide direct evidence that NOS-1 acts in a site-specific manner to promote cardiac vagal neurotransmission and bradycardia. NOS-1 gene transfer to the guinea pig right atrium increased protein expression and NOS-1 immunolocalization in cholinergic ganglia. It also increased the release of acetylcholine and enhanced the heart rate (HR) response to vagal nerve stimulation (VNS) in vitro and in vivo. NOS inhibition normalized the HR response to VNS in the NOS-1-treated group compared with the control groups (enhanced green fluorescent protein and sham) in vitro. In contrast, an acetylcholine analogue reduced HR to the same extent in all groups before and during NOS inhibition. These results demonstrate that NOS-1-derived NO acts presynaptically to facilitate vagally induced bradycardia and that upregulation of NOS-1 via gene transfer may provide a novel method for increasing cardiac vagal function.

Estrogen modulation of eNOS activity and its association with caveolin-3 and calmodulin in rat hearts.

Previous studies have shown that estrogen modulation of endothelial nitric oxide (NO) synthase (eNOS) may confer protection against heart disease. Here, we demonstrate an association between reductions in baroreflex-mediated bradycardia and in cardiac NOS activity in ovariectomized (Ovx) rats compared with controls. The latter resulted, at least in part, from a reduction in cardiac eNOS protein. eNOS-derived NO and its biological effects are determined by the levels of eNOS protein and by eNOS catalytic activity; the latter is regulated partly through the dynamic interaction with an inhibitory protein (caveolin) and a stimulatory protein (calmodulin). The association of eNOS immunoprecipitated with caveolin-3 and calmodulin was examined. Caveolin-3 and calmodulin binding with eNOS was increased and decreased, respectively, in Ovx rats. 17 beta-Estradiol replacement restored, to within normal levels, the baroreflex-mediated bradycardic responses along with eNOS activity, eNOS expression, and the association of eNOS with caveolin-3 and calmodulin. Our findings may help to elucidate the molecular mechanism underlying the favorable effects of estrogen on cardiac responses to baroreflex activation.

Overexpression of eNOS in NTS causes hypotension and bradycardia in vivo.

The role of nitric oxide (NO) in the brain in the control of blood pressure and the sympathetic nervous system is debated. This study examined the effect of overexpression of endothelial NO synthase (eNOS) in the nucleus tractus solitarii (NTS) on blood pressure in conscious rats. Adenovirus vectors encoding either eNOS (AdeNOS) or ss-galactosidase were transfected into the NTS in vivo. In the AdeNOS-treated rats, the local expression of eNOS in the NTS was confirmed by immunohistochemical staining and Western blot analysis for the eNOS protein and by increased production of nitrite/nitrate in the NTS measured by in vivo microdialysis. Blood pressure and heart rate, monitored by the use of a radiotelemetry system in a conscious state, were significantly decreased in the AdeNOS-treated group at day 5 to day 10 after the gene transfer. Urinary norepinephrine excretion also was decreased at day 7 after the gene transfer in the AdeNOS-treated group. Our results indicate that overexpression of eNOS in the NTS decreases blood pressure, heart rate, and sympathetic nerve activity in conscious rats.