omega-Conotoxin GVIA and desipramine insensitive norepinephrine efflux from cardiac sympathetic nerve terminal

The organ-protective effect of N-type Ca(2+) channel blockade

The six subtypes of voltage-dependent Ca(2+) channels (VDCCs) mediate a wide range of physiological responses. N-type VDCCs (NCCs) were originally identified as a high voltage-activated Ca(2+) channel selectively blocked by omega-conotoxin (ω-CTX)-GVIA. Predominantly localized in the nervous system, NCCs are key regulators of neurotransmitter release. Both pharmacological blockade with ω-CTX-GVIA and, more recently, mice lacking CNCNA1B, encoding the α1B subunit of NCC, have been used to assess the physiological and pathophysiological functions of NCCs, revealing in part their significant roles in sympathetic nerve activation and nociceptive transmission. The evidence now available indicates that NCCs are a potentially useful therapeutic target for the treatment of several pathological conditions. Efforts are therefore being made to develop effective NCC blockers, including both synthetic ω-CTX-GVIA derivatives and small-molecule inhibitors. Cilnidipine, for example, is a dihydropyridine L-type VDCC blocking agent that also possesses significant NCC blocking ability. As over-activation of the sympathetic nervous system appears to contribute to the pathological processes underlying cardiovascular, renal and metabolic diseases, NCC blockade could be a useful approach to treating these ailments. In this review article, we provide an overview of what is currently known about the physiological and pathophysiological activities of NCCs and the potentially beneficial effects of NCC blockade in several disease conditions, in particular cardiovascular diseases.

Accumbal noradrenaline that contributes to the alpha-adrenoceptor-mediated release of dopamine from reserpine-sensitive storage vesicles in the nucleus accumbens is derived from alpha-methyl-para-tyrosine-sensitive pools

Alpha-adrenoceptors in the nucleus accumbens are known to inhibit accumbal dopamine release from reserpine-sensitive pools. The aim of this study was to test our previously reported hypothesis that accumbal noradrenaline that controls the dopamine release from these storage vesicles is derived from alpha-methyl-para-tyrosine-sensitive pools. The sensitivity of accumbal alpha-adrenoceptors to noradrenergic agents depends on the amount of noradrenaline that is available in the synapse. In case the synaptic noradrenaline levels decrease, the conformation of alpha-adrenoceptors changes into a state that makes these receptors more sensitive to its agonists. The effects of alpha-methyl-para-tyrosine, respectively reserpine, on the alpha-adrenoceptor-agonist-induced changes of accumbal dopamine release were investigated. Alpha-methyl-para-tyrosine, but not reserpine, made accumbal postsynaptic alpha-adrenoceptors more sensitive to phenylephrine. These results indicate that noradrenaline that inhibits the release of dopamine from reserpine-sensitive storage vesicles, via stimulation of accumbal postsynaptic alpha-adrenoceptors, is derived from alpha-methyl-para-tyrosine-sensitive pools. The clinical impact of these data is discussed.

Effects of ketamine on exocytotic and non-exocytotic noradrenaline release

To characterise ketamine-induced sympathomimetic action, we examined the effects of ketamine on in vivo cardiac sympathetic nerve endings function. Using adult cats given anaesthesia with pentobarbital, dialysis probes were implanted in the left ventricular myocardium and dialysate noradrenaline (NA) concentrations were measured as an indicator of NA output at the cardiac sympathetic nerve endings. Ketamine was locally administered through the dialysis probe, and dialysate NA responses were obtained in the following conditions. (1) In the resting state, ketamine (10 mM) increased dialysate NA concentration. This increase in dialysate NA was not altered by addition of omega-conotoxin GVIA (N-type Ca(2+) channel blocker) or desipramine (membrane NA uptake blocker). (2) Sympathetic activation by electrical stimulation of the stellate ganglia (ES-SG; exocytotic NA release): ES-SG caused an increase in dialysate NA, which was further augmented by addition of desipramine. During co-administration of desipramine and ketamine, dialysate NA response to ES-SG was smaller than with desipramine alone. Further, there was no significant difference in the dialysate NA response to ES-SG between ketamine and ketamine + desipramine. These data suggested that both exocytosis and NA uptake function were impaired by ketamine. (3) Non-exocytotic NA release by ouabain: ouabain caused increases in dialysate NA. These increases in dialysate NA were suppressed by ketamine, which impaired the membrane outward NA transport evoked by ouabain. We conclude that ketamine impaired exocytotic and non-exocytotic NA release. However, ketamine spontaneously evoked NA efflux that was independent of exocytosis and insensitive to NA transporter.

Modulatory effects of ketamine on catecholamine efflux from in vivo cardiac sympathetic nerve endings in cats

With the use of the microdialysis technique, we examined the modulatory effect of ketamine on catecholamine efflux from in vivo cardiac sympathetic nerve endings. A dialysis probe was implanted in the left ventricular myocardium, and dialysate norepinephrine (NE) levels in anesthetized cats were measured with liquid chromatogram-electrical detection. A 60-min occlusion of the left anterior descending coronary artery caused increases in dialysate NE levels. Through the dialysis probe, locally applied ketamine (10 mM) augmented the dialysate NE responses to coronary occlusion in the presence and absence of desipramine (membrane NE transport blocker). Thus, the ketamine-induced NE increment is not mediated through the neuronal NE transporter. The sympathomimetic action of ketamine may augment the NE efflux evoked by myocardial ischemia.

Effects of ketamine on in vivo cardiac sympathetic nerve endings

Using the dialysis technique, we examined the effect of ketamine on dialysate norepinephrine (NE) levels in the myocardial interstitial space in anesthetized cats. Dialysis probes were implanted in the left ventricular myocardium, and we measured the dialysate NE levels serving as an indicator of NE output at the cardiac sympathetic nerve endings. During local administration of ketamine (10 mM), we examined the time-course of the change in dialysate NE levels and the dialysate NE response to coronary occlusion. Dialysate NE levels significantly increased from 39+/-7 pg/ml at control to 133+/-22 pg/ml 30 min after beginning the ketamine administration. Addition of either omega-conotoxin GVIA (N-type calcium channel blocker) at 10 microg/kg intravenously or desipramine (neuronal NE transport blocker) at 100 microM did not inhibit the increment in dialysate NE evoked by ketamine. These findings suggest that the increase in dialysate NE evoked by ketamine is dependent neither on the activity of NE exocytosis nor on the neuronal NE transport. Left descending coronary artery occlusion evoked increments in dialysate NE. The addition of ketamine augmented the dialysate NE response to coronary occlusion. A ketamine-induced increment in dialysate NE might occur as a consequence of NE exocytosis independent or membrane NE transport insensitive efflux of NE.

Functional disorders of the sympathetic nervous system in mice lacking the alpha 1B subunit (Cav 2.2) of N-type calcium channels

N-type voltage-dependent Ca(2+) channels (VDCCs), predominantly localized in the nervous system, have been considered to play an essential role in a variety of neuronal functions, including neurotransmitter release at sympathetic nerve terminals. As a direct approach to elucidating the physiological significance of N-type VDCCs, we have generated mice genetically deficient in the alpha(1B) subunit (Ca(v) 2.2). The alpha(1B)-deficient null mice, surprisingly, have a normal life span and are free from apparent behavioral defects. A complete and selective elimination of N-type currents, sensitive to omega-conotoxin GVIA, was observed without significant changes in the activity of other VDCC types in neuronal preparations of mutant mice. The baroreflex response, mediated by the sympathetic nervous system, was markedly reduced after bilateral carotid occlusion. In isolated left atria prepared from N-type-deficient mice, the positive inotropic responses to electrical sympathetic neuronal stimulation were dramatically decreased compared with those of normal mice. In contrast, parasympathetic nervous activity in the mutant mice was nearly identical to that of wild-type mice. Interestingly, the mutant mice showed sustained elevation of heart rate and blood pressure. These results provide direct evidence that N-type VDCCs are indispensable for the function of the sympathetic nervous system in circulatory regulation and indicate that N-type VDCC-deficient mice will be a useful model for studying disorders attributable to sympathetic nerve dysfunction.

Liquid chromatographic determination of myocardial interstitial epinephrine

This study describes a high-performance liquid chromatographic method with electrochemical detection (HPLC-ED) for monitoring of epinephrine (Epi) in the myocardial interstitial space. The in vitro detection limit for Epi was 200 fg in a 50-microl injection. Using a cardiac dialysis technique, 60-microl dialysates were sampled from the myocardial interstitial space (6-min fractions). After an alumina procedure, the dialysate Epi concentration was measured using the HPLC-ED system. Although the basal Epi concentration was undetectable, local administration of desipramine increased Epi concentration of the dialysate to 38.1+/-18.5 pg/ml. This system affords a new possibility for estimating myocardial interstitial Epi level.