The effects of varying Mg2+ ion concentrations on contractions to the cotransmitters ATP and noradrenaline in the rat vas deferens

The vas deferens responds to a single electrical pulse with a biphasic contraction caused by cotransmitters ATP and noradrenaline. Removing Mg²⁺ (normally 1.2 mM) from the physiological salt solution (PSS) enhances the contraction. This study aimed to determine the effect of Mg²⁺ concentration on nerve cotransmitter-mediated contractions. Rat vasa deferentia were sequentially bathed in increasing (0, 1.2, 3 mM) or decreasing (3, 1.2, 0 mM) Mg²⁺ concentrations. At each concentration a single field pulse was applied, and the biphasic contraction recorded. Contractions to exogenous noradrenaline 10 μM and ATP 100 μM were also determined. The biphasic nerve-mediated contraction was elicited by ATP and noradrenaline as NF449 (10 μM) and prazosin (100 nM) completely prevented the respective peaks. Taking the contractions in normal PSS (Mg²⁺ 1.2 mM) as 100%, lowering Mg²⁺ to 0 mM enhanced the ATP peak to 170 ± 7% and raising Mg²⁺ to 3 mM decreased it to 39 ± 3%; the noradrenaline peak was not affected by lowering Mg²⁺ to 0 mM (97 ± 3%) but was decreased to 63 ± 4% in high Mg²⁺ (3 mM). Contractions to exogenous ATP, but not noradrenaline, were increased in Mg²⁺ 0 mM and both were inhibited with Mg²⁺ 3 mM. Changing Mg²⁺ concentration affects the contractions elicited by the cotransmitters ATP and noradrenaline. The greatest effects were to potentiate the contraction to ATP in Mg²⁺ 0 mM and to inhibit the contraction to both ATP and noradrenaline in high Mg²⁺. Future publications should clearly justify any decision to vary the magnesium concentration from normal (1.2 mM) values.

Ero1-Mediated Reoxidation of Protein Disulfide Isomerase Accelerates the Folding of Cone Snail Toxins

Disulfide-rich peptides are highly abundant in nature and their study has provided fascinating insight into protein folding, structure and function. Venomous cone snails belong to a group of organisms that express one of the largest sets of disulfide-rich peptides (conotoxins) found in nature. The diversity of structural scaffolds found for conotoxins suggests that specialized molecular adaptations have evolved to ensure their efficient folding and secretion. We recently showed that canonical protein disulfide isomerase (PDI) and a conotoxin-specific PDI (csPDI) are ubiquitously expressed in the venom gland of cone snails and play a major role in conotoxin folding. Here, we identify cone snail endoplasmic reticulum oxidoreductin-1 (Conus Ero1) and investigate its role in the oxidative folding of conotoxins through reoxidation of cone snail PDI and csPDI. We show that Conus Ero1 preferentially reoxidizes PDI over csPDI, suggesting that the reoxidation of csPDI may rely on an Ero1-independent molecular pathway. Despite the preferential reoxidation of PDI over csPDI, the combinatorial effect of Ero1 and csPDI provides higher folding yields than Ero1 and PDI. We further demonstrate that the highest in vitro folding rates of two model conotoxins are achieved when all three enzymes are present, indicating that these enzymes may act synergistically. Our findings provide new insight into the generation of one of the most diverse classes of disulfide-rich peptides and may improve current in vitro approaches for the production of venom peptides for pharmacological studies.

Gene expression profiles of ion channels and receptors in mouse resistance arteries: Effects of cell type, vascular bed, and age

OBJECTIVE

Receptors and ion channels of smooth muscle cells (SMCs) and endothelial cells (ECs) are integral to the regulation of vessel diameter and tissue blood flow. Physiological roles of ion channels and receptors in skeletal muscle and mesenteric arteries have been identified; however, their gene expression profiles are undefined. We tested the hypothesis that expression profiles for ion channels and receptors governing vascular reactivity vary with cell type, vascular bed, and age.

METHODS

Mesenteric and superior epigastric arteries were dissected from Old (24-26 months) and Young (3-6 months) C57BL/6J mice. ECs and SMCs were collected for analysis with custom qRT-PCR arrays to determine expression profiles of 80 ion channel and receptor genes. Bioinformatics analyses were applied to gain insight into functional interactions.

RESULTS

We identified 68 differences in gene expression with respect to cell type, vessel type, and age. Heat maps illustrate differential expression, and distance matrices predict patterns of coexpression. Gene networks based upon protein-protein interaction datasets and KEGG pathways illustrate biological processes affected by specific differences in gene expression.

CONCLUSIONS

Differences in gene expression profiles are most pronounced between microvascular ECs and SMCs with subtle variations between vascular beds and age groups.

Suppression of peripheral sympathetic activity underlies protease-activated receptor 2-mediated hypotension

Protease-activated receptor (PAR)-2 is expressed in endothelial cells and vascular smooth muscle cells. It plays a crucial role in regulating blood pressure via the modulation of peripheral vascular tone. Although some reports have suggested involvement of a neurogenic mechanism in PAR-2-induced hypotension, the accurate mechanism remains to be elucidated. To examine this possibility, we investigated the effect of PAR-2 activation on smooth muscle contraction evoked by electrical field stimulation (EFS) in the superior mesenteric artery. In the present study, PAR-2 agonists suppressed neurogenic contractions evoked by EFS in endothelium-denuded superior mesenteric arterial strips but did not affect contraction elicited by the external application of noradrenaline (NA). However, thrombin, a potent PAR-1 agonist, had no effect on EFS-evoked contraction. Additionally, ω-conotoxin GVIA (CgTx), a selective N-type Ca²⁺ channel (I_Ca-N) blocker, significantly inhibited EFS-evoked contraction, and this blockade almost completely occluded the suppression of EFS-evoked contraction by PAR-2 agonists. Finally, PAR-2 agonists suppressed the EFS-evoked overflow of NA in endothelium-denuded rat superior mesenteric arterial strips and this suppression was nearly completely occluded by ω-CgTx. These results suggest that activation of PAR-2 may suppress peripheral sympathetic outflow by modulating activity of I_Ca-N which are located in peripheral sympathetic nerve terminals, which results in PAR-2-induced hypotension.

The vasorelaxant effect of pituitary adenylate cyclase activating polypeptide and vasoactive intestinal polypeptide in isolated rat basilar arteries is partially mediated by activation of nitrergic neurons

The structurally related neuropeptides pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are recognised by two G protein-coupled receptors, termed VPAC(1)-R and VPAC(2)-R, with equal affinity. PACAP and VIP have previously been shown to relax cerebral arteries in an endothelium-independent manner. The aim of the present study was to test if intramural neurons are involved in the mediation of PACAP/VIP-induced vasodilatory responses. Therefore, the vascular tone of isolated rat basilar arteries was measured by means of a myograph. The vasorelaxing effect of PACAP was assessed in arteries precontracted by serotonin in the absence or presence of different test compounds known to selectively inhibit certain signaling proteins. The vasorelaxant effect of PACAP could be significantly reduced by the inhibitor of neuronal N-type calcium channels omega-conotoxin GVIA (omega-CgTx), as well as by 3-bromo-7-nitroindazole (3Br-7-Ni), an inhibitor of the neuronal nitric oxide-synthase (nNOS). The localization of N-type calcium channels and VPAC-Rs within the rat basilar artery was investigated by confocal laser scanning microscopy using omega-CgTx- and VIP-analogs labelled with fluorescent dyes. These findings suggest that activation of intramural neurons may represent an important effector mechanism for mediation of the vasorelaxant PACAP-response.

Effects of omega-conotoxin GVIA and diltiazem on double peaked vasoconstrictor responses to periarterial electric nerve stimulation in isolated canine splenic artery

The actions of omega-conotoxin (omega-CTX) and diltiazem on adrenergic and purinergic components of double peaked vasoconstrictor responses to periarterial nerve stimulation have been investigated in the isolated, perfused canine splenic arterial preparation. Double peaked vasoconstrictions (biphases of vasoconstrictors) were consistently observed in the conditions of 30 s trains of pulses at 1 - 10 Hz frequencies. omega-CTX (1 - 30 nM) produced similar inhibitory effects on the first phase and second phase responses in a dose-related manner. Thirty nM omega-CTX almost completely inhibited the biphasic vasoconstrictions at any used frequencies but did not affect the vasoconstrictor responses to exogenous applied ATP (0.01 - 1 micromol) and noradrenaline (0.03 - 3 nmol). Intraluminal application of a large dose of diltiazem (3 - 10 microM) also produced a dose-dependent inhibitory effect on biphasic vasoconstrictions at any used frequencies. Three microM diltiazem exerted rather a larger inhibitory effect on the second phase than the first phase response at low frequencies (1 - 3 Hz), but a similar inhibition on first and second phasic responses at high frequencies (6 - 10 Hz). An extremely high dose of diltiazem (10 microM) almost completely inhibited the biphasic vasoconstrictor responses to nerve stimulation, and slightly inhibited the contractile responses to exogenous applied ATP (0.01 - 1 micromol) and noradrenaline (0.03 - 3 nmol). The present results indicate that omega-CTX selectively acts prejunctionally to inhibit the release of transmitters from sympathetic nerve terminals, and omega-CTX-sensitive calcium channels may produce a parallel controlling of purinergic and adrenergic components of sympathetic cotransmission. A large dose of diltiazem has inhibitory effects on both prejunctional and postjunctional sympathetic co-transmission. British Journal of Pharmacology (2000) 129, 47 - 52

Postural hypotension following N-type Ca2+ channel blockade is amplified in experimental hypertension

OBJECTIVE

To determine the relative importance of the cardiac and vascular sympathetic components of the orthostatic response to 90 degrees head-up tilt after N-type calcium-channel blockade in normotensive (sham renal cellophane wrap) and hypertensive (renal wrap) conscious rabbits.

METHODS

The effects of N-type calcium-channel blockade with omega-conotoxin GVIA (omega-CTX, 10 microg/kg i.v. bolus) were assessed in the absence or presence of cardiac block by propranolol and methscopolamine. These were contrasted with the effects of alpha1-adrenoceptor antagonism (prazosin 0.5 mg/kg i.v. bolus, in the presence of cardiac block) or ganglion blockade (mecamylamine 4 mg/kg i.v. bolus).

RESULTS

In vehicle (0.9% saline) treatment groups, the response to tilt consisted of a small pressor effect (4 +/- 2 and 7 +/- 1 mmHg) and tachycardia (29 +/- 6 and 17 +/- 6 beats/min) in sham (n = 6) and wrap (n = 5) rabbits, respectively. After prazosin administration (with cardiac block), there were significant falls in MAP of 3 +/- 1 and 7 +/- 2 mmHg in sham (n = 7) and wrap (n = 6) rabbits, respectively, in response to tilt omega-CTX caused postural hypotensive responses of 8 +/- 2 and 13 +/- 2 mmHg in sham (n = 6) and wrap (n = 7) rabbits, respectively, and 7 +/- 1 and 14 +/- 2 mmHg in sham (n = 7) and wrap (n = 7) rabbits with prior cardiac block. Similarly, mecamylamine caused falls in MAP of 8 +/- 1 and 10 +/- 2 mmHg in response to tilt in sham (n = 6) and wrap (n = 9) animals, respectively.

CONCLUSION

Sympathetic vasoconstrictor effectors are primarily responsible for maintaining blood pressure during tilt in conscious rabbits. The postural hypotension caused by sympatholytic agents is about double in hypertensive rabbits, and N-type calcium-channel blockade is as effective as ganglion blockade at inducing this syndrome.

Significant role of neuronal non-N-type calcium channels in the sympathetic neurogenic contraction of rat mesenteric artery

1. The possible involvement of pre-junctional non-N-type Ca2+ channels in noradrenaline (NA)-mediated neurogenic contraction by electrical field stimulation (EFS) was examined pharmacomechanically in the isolated rat mesenteric artery. 2. EFS-generated contraction of endothelium-denuded mesenteric artery was frequency-dependent (2 - 32 Hz) and was abolished by tetrodotoxin (TTX, 1 microM), guanethidine (5 microM) or prazosin (100 nM), indicating that NA released from sympathetic nerve endings mediates the contractile response. 3. NA-mediated neurogenic contractions to lower frequency stimulations (2 - 8 Hz) were almost abolished by an N-type Ca2+ channel blocker, omega-conotoxin-GVIA (1 microM) whereas the responses to higher frequency stimulations (12 - 32 Hz) were less sensitive to omega-conotoxin-GVIA. The omega-conotoxin-GVIA-resistant component of the contractile response to 32 Hz stimulation was inhibited partly (10 - 20%) by omega-agatoxin-IVA (10 - 100 nM; concentrations which are relatively selective for P-type channels) and to a greater extent by omega-agatoxin-IVA (1 microM) and omega-conotoxin-MVIIC (3 microM), both of which block Q-type channels at the concentrations used. 4. omega-Agatoxin-IVA (10 - 100 nM) alone inhibited 32 Hz EFS-induced contraction by 10 approximately 20% whereas omega-conotoxin-MVIIC (3 microM) alone inhibited the response by approximately 60%. 5. These omega-toxin treatments did not affect the contractions evoked by exogenously applied NA. 6. These findings show that P- and Q-type as well as N-type Ca2+ channels are involved in the sympathetic neurogenic vascular contraction, and suggest the significant role of non-N-type Ca2+ channels in NA release from adrenergic nerve endings when higher frequency stimulations are applied to the nerve.

Role of N-type calcium channels in autonomic neurotransmission in guinea-pig isolated left atria

1. Calcium entry via neuronal calcium channels is essential for the process of neurotransmission. We investigated the calcium channel subtypes involved in the operation of cardiac autonomic neurotransmission by examining the effects of selective calcium channel blockers on the inotropic responses to electrical field stimulation (EFS) of driven (4 Hz) guinea-pig isolated left atria. In this tissue, a previous report (Hong & Chang, 1995) found no evidence for N-type channels involved in the vagal negative inotropic response and only weak involvement in sympathetic responses. 2. The effects of cumulative concentrations of the selective N-type calcium channel blocker, omega-conotoxin GVIA (GVIA; 0.1-10 nM) and the non-selective N-, P/Q-type calcium channel blocker, omega-conotoxin MVIIC (MVIIC; 0.01-10 nM) were examined on the positive (with atropine, 1 microM present) and negative (with propranolol, 1 microM and clonidine, 1 microM present) inotropic responses to EFS (eight trains, each train four pulses per punctate stimulus). 3. GVIA caused complete inhibition of both cardiac vagal and sympathetic inotropic responses to EFS. GVIA was equipotent at inhibiting positive (pIC50 9.29+/-0.08) and negative (pIC50 9.13+/-0.17) inotropic responses. MVIIC also mediated complete inhibition of inotropic responses to EFS and was 160 and 85 fold less potent than GVIA at inhibiting positive (pIC50 7.08+/-0.10) and negative (pIC50 7.20+/-0.14) inotropic responses, respectively. MVIIC was also equipotent at inhibiting both sympathetic and vagal responses. 4. Our data demonstrates that N-type calcium channels account for all the calcium current required for cardiac autonomic neurotransmission in the guinea-pig isolated left atrium.

Endogenous angiotensin II and bradykinin delay and attenuate the hypotension after N-type calcium channel blockade in conscious rabbits

The effects of N-type calcium channel inhibition with omega-conotoxin GVIA (omega-CTX) on cardiovascular parameters and vagally mediated autonomic reflexes and the role of the renin-angiotensin system were assessed in conscious rabbits. Omega-CTX (10 microg/kg, i.v.) resulted in hypotension, tachycardia, and attenuation of the sympathetic and vagal components of the baroreceptor-heart rate reflex (baroreflex). In the control group (no pretreatment), the peak decrease in mean arterial pressure (MAP) of 13 +/- 3 mm Hg from 72 +/- 2 mm Hg occurred after 33 +/- 3 min, with a corresponding tachycardia of 80 +/- 20 beats/min (n = 6). The tachycardia was due to vagal withdrawal, as a similar increase in heart rate (84 +/- 8 beats/min) after omega-CTX was observed after pretreatment with the beta-adrenoceptor antagonist, propranolol (n = 6). Angiotensin-converting enzyme (ACE) inhibition with enalaprilat revealed a larger, more rapid decrease in MAP in response to omega-CTX (-19 +/- 4 mm Hg from 65 +/- 1 mm Hg after 18 +/- 2 min; n = 6) compared with the control group. Similar larger decreases in MAP were also observed in the presence of the AT1-receptor antagonist, losartan, or the bradykinin B2 receptor antagonist, HOE-140 (n = 5-6). Pretreatment with enalaprilat, losartan, or HOE-140 caused a 50% decrease in the reflex tachycardia after omega-CTX compared with that observed in the control group, and omega-CTX caused a greater attenuation of the vagal component of the baroreflex and a decrease in the bradycardia evoked by the Bezold-Jarisch-like reflex. Also, there was a significant decrease in the bradycardia induced by the nasopharyngeal reflex after omega-CTX in the presence of ACE inhibition and HOE-140. Thus in the conscious rabbit, angiotensin II and bradykinin have a role in attenuating and slowing the hypotensive effect of N-type calcium channel inhibition. Vagolytic effects of omega-CTX on the baroreflex are augmented, and on other vagal reflexes are unmasked, via inhibition of the renin-angiotensin system. The complexity and mechanism of the interaction between N-type calcium channels and the renin-angiotensin system remain to be elucidated.

Prolonged cardiovascular effects of the N-type Ca2+ channel antagonist omega-conotoxin GVIA in conscious rabbits

omega-Conotoxin GVIA (omega-CTX) is an N-type Ca2+ channel antagonist that is considered to be only partially reversible in vitro. In vivo, its effects after 24 h are unknown. To assess the duration of action of this peptide in vivo, the effects of a single intravenous injection of omega-CTX on mean arterial pressure (MAP), heart rate (HR), postural adaptation, and the baroreflex were investigated in conscious rabbits. MAP, HR, the baroreflex induced by i.v. glyceryl trinitrate (0.4-20 micrograms/kg) and phenylephrine (0.1-15 micrograms/kg) and orthostatic responses to 1 min 90 degrees head-up tilt were assessed before (0 h) and 2-168 h after administration of omega-CTX (10 micrograms/kg i.v. bolus: n = 6-9) or vehicle (0.9% saline; n = 6). Acute phase I: By 2 h after omega-CTX administration, MAP had decreased from 75 +/- 3 mm Hg to 60 +/- 2 mm Hg; HR increased from 220 +/- 7 beats/min to 249 +/- 5 beats/min (n = 9). There was marked attenuation of the baroreflex curve (HR range decreasing by 61%). By 24 h. MAP and HR had returned to control values, but the HR range was still 18% less than that of control. Phase II: MAP and HR then decreased steadily over the next 96 h to significantly lower values by 120 h after omega-CTX administration (delta-8 +/- 2 mm Hg and -29 +/- 2 beats/min, respectively; n = 6). Thereafter, MAP and HR values increased and by 168 h these parameters, and the baroreflex, were similar to control values. In response to 90 degrees tilt, there was no change in MAP at 0 h; however, 1 h after omega-CTX, significant postural hypotension was observed with decreases of 14 +/- 1 mm Hg(n = 9). Smaller orthostatic responses were still observed 48 h after omega-CTX administration: however, by 72 h, head-up tilt no longer induced a significant change in MAP. In the vehicle-treatment group, there were no changes in cardiovascular parameters during 0-168 h. Thus omega-CTX (10 micrograms/kg i.v.) causes acute hypotension, as well as postural hypotension, and has sympatholytic and vagolytic effects that are mostly reversed after 48 h in the conscious rabbit. However, a second hypotensive and bradycardic phase lasting a further 96 h ensues, suggesting that other prolonged effects from central neural or hormonal mechanisms or fluid shifts may occur.

New insights into vascular reactivity: from altered structure to neural control

1. The present review covers two aspects of the author's research into the pharmacology of vascular reactivity of isolated vessels and in the intact circulation. First, how 'normal' reactivity is altered by injury or disease and, second, how novel drugs have allowed insight into the role of the cotransmitter neuropeptide Y and 'N' type calcium channels in neurotransmitter release. 2. Acute endothelium removal in the femoral artery of the anaesthetized dog confirmed the obligatory role of these cells in the dilatation response to intra-arterial acetylcholine (ACh). After 4 weeks, conduit arteries respond with a thickened neointima following acute endothelial injury but, provided macrophage-derived foam cells are absent, the artery relaxes normally to ACh. 3. In the dog coronary vasculature, stable collateral arteries have a marked neointima of non-contractile smooth muscle cells that are lined with endothelium. Reactivity to vasodilator stimuli is normal while that to vasoconstrictor stimuli is impaired. 4. In the conscious rabbit, superficial femoral artery (SFA) occlusion stimulates profound angiogenesis but, despite these changes to the hindlimb vasculature, reactivity to vasodilator and vasoconstrictor agents from day 1 to 6 months following SFA is unaltered. 5. Endothelial dysfunction is discussed in relation to hypertension, hypercholesterolaemia and congestive heart failure. 6. The novel "N' type calcium channel antagonist omega-conotoxin GVIA, was used to explore the role of "N' type channels in cardiac and vascular neurotransmitter release in conscious rabbits. 7. The novel putative Y 1-selective neuropeptide Y antagonist 1229U91 was shown to inhibit nerve-mediated contractions of isolated mesenteric, but not femoral, artery segments in the rat. This regional difference in a possible cotransmitter role of the peptide is discussed.

Differential effects of omega-conotoxin GVIA on cholinergic and non-cholinergic secretomotor neurones in the guinea-pig small intestine

1. Ussing chambers were used to study the effects of the specific N-type Ca2+ channel antagonist, omega-conotoxin GVIA, on neurally evoked secretion across isolated submucosa/mucosa preparations from the small intestine of the guinea-pig. 2. Cholinergic and non-cholinergic neurones were stimulated with 10 microM dimethylphenylpiperazinium (DMPP). Non-cholinergic secretomotor neurones were preferentially stimulated with 100 nM 5-hydroxytryptamine (5-HT), while cholinergic secretomotor neurones were preferentially stimulated with 3 microM 5-HT in the presence of the 5-HT2 receptor antagonist ketanserin (30 nM). 3. omega-Conotoxin GVIA (1 nM-1 microM) depressed the secretion evoked by DMPP in a concentration-dependent manner, but a substantial residual response was observed. Hyoscine (100 nM) significantly depressed secretion evoked by DMPP, but did not prevent further depression of secretion by omega-conotoxin GVIA. 4. The toxin was substantially more effective when the non-cholinergic secretomotor neurones were preferentially activated with 100 nM 5-HT, with a decrease in the response of more than 75% of the control value in the presence of 1 microM omega-conotoxin GVIA. 5. omega-Conotoxin GVIA (1 microM) was relatively ineffective against secretion evoked by preferential activation of cholinergic secretomotor nuerones with 3 microM 5-HT in the presence of 30 nM ketanserin, inhibiting the response by less than 33%. However, this inhibition was significant. Both 100 nM hyoscine and 300 nM tetrodotoxin abolished this effect of omega-conotoxin GVIA. 6. It is concluded that N-type Ca2+ channels play a major role in transmitter release from non-cholinergic secretomotor neurones, but are not important for release from cholinergic secretomotor neurones in the guinea-pig small intestine.

Effects of N-, P- and Q-type neuronal calcium channel antagonists on mammalian peripheral neurotransmission

1. The effects of N-, P- and Q-type neuronal voltage-operated calcium (Ca2+) channel antagonists on neurotransmission were determined in a range of cardiovascular and urogenital tissues, as well as the diaphragm, isolated from rat or mouse. 2. The pharmacological tools chosen were omega-conotoxin GVIA (CTX GVIA), a selective N-type Ca2+ channel antagonist, the P-type channel blocker (< or = 100 nM) omega-agatoxin IVA (AGA IVA) and omega-conotoxin MVIIC (CTX MVIIC), a non-selective antagonist of N-, P- and Q-type channels. The effects of these antagonists on nerve-mediated responses were assessed in right atria, vasa deferentia, phrenic nerve-hemidiaphragms and small mesenteric arteries. 3. Rat mesenteric artery contractile responses to perivascular nerve stimulation were concentration-dependently inhibited by CTX GVIA (1-10 nM); inhibition was 92% with 10 nM. CTX MVIIC was > 100 fold less potent and only caused an inhibition of 46% at the highest concentration (1000 nM). AGA IVA (100 nM) had no effect. 4. In rat vas deferens stimulated at 0.05 Hz, CTX GVIA (10 nM) completely inhibited the twitch response and CTX MVIIC, about 100 fold less potent, caused total inhibition at 1000 nM. AGA IVA did not affect the twitch. In rat preparations stimulated at 20 Hz, a CTX GVIA-resistant (< or = 1000 nM) twitch response of 25% was apparent which could be blocked by 1000 nM AGA IVA or CTX MVIIC. In mouse vas deferens (20 Hz stimulation), CTX GVIA 10 nM caused an 87% inhibition of the twitch, the remainder being resistant to CTX GVIA, 100 nM. CTX MVIIC was only 10 fold less potent than CTX GVIA and completely inhibited the response at 1000 nM. AGA IVA (100 nM) inhibited the twitch by 55%. 5. The twitch response of the mouse phrenic nerve-hemidiaphragm was concentration-dependently inhibited by AGA IVA (1-100 nM); inhibition was 92% at 100 nM. CTX MVIIC was about 10 fold less potent than AGA IVA with an inhibition of 80% at 1000 nM. CTX GVIA was without effect. In the rat diaphragm preparation, AGA IVA (< or = 100 nM) and CTX GVIA (< or = 1000 nM) had little effect on the twitch response. CTX MVIIC (1000 nM) inhibited the twitch by 57%. 6. In rat and mouse right atria, sympathetic responses were concentration-dependently inhibited by CTX GVIA with almost complete block at 10-100 nM. CTX MVIIC was 100 fold less potent and caused complete inhibition at 1000 nM in the mouse preparation. AGA IVA did not affect atrial sympathetic responses. 7. These data suggest that N-type Ca2+ channels predominate in the control of sympathetic transmission in the mesenteric artery, vas deferens and right atrium. In the mouse vas deferens (and rat tissue at high stimulus frequency), P- and Q-type channels also mediate Ca2+ influx. P- and Q-type Ca2+ channels control neurosecretion at the motor endplate, with no role for N type channels.