An investigation of the actions of the calcium entry facilitator Bay K 8644 on the rat vas deferens

Prejunctional actions of K+ channel blockers in rat vas deferens

In studies of isometric contractions in prostatic portions of rat vas deferens evoked by single pulse electrical stimulation, the K+ channel blockers 4-aminopyridine, tetraethylammonium and charybdotoxin, but not apamin, significantly reduced the prejunctional inhibitory potency and the maximum inhibitory effect of the alpha 2-adrenoceptor agonist xylazine. The protein kinase C activator phorbol dibutyrate had similar effects to 4-aminopyridine against xylazine. However, 4-aminopyridine, tetraethylammonium, charybdotoxin and phorbol dibutyrate, but not apamin, significantly increased the magnitude of the isometric contraction to a single stimulus. 4-Aminopyridine and phorbol dibutyrate significantly reduced, while tetraethylammonium did not affect, isometric contractions to noradrenaline, and 4-aminopyridine failed to affect contractions to alpha,beta-methylene-ATP, so that the effects of these agents on the isometric contraction to a single stimulus were presumably by a prejunctional action. The Ca2+ entry facilitator Bay K 8644 (1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyrid ine carboxylic acid methylester) increased stimulation-evoked contractions by a postjunctional action and reduced the inhibitory effects of xylazine. When the isometric contraction following 4-aminopyridine was reduced by decreasing the stimulation voltage or by reducing the Ca2+ concentration from 2.5 to 0.9 mM, 4-aminopyridine significantly reduced the potency of xylazine. However, tetraethylammonium and Bay K 8644 failed to affect the inhibitory potency of xylazine in low Ca2+. It is concluded that the K+ channel blocker 4-aminopyridine reduces the prejunctional inhibitory potency of xylazine, and this action is independent of increased neurotransmitter release. These results suggest that prejunctional alpha 2-adrenoceptor-mediated inhibition in rat vas deferens involves K+ channels sensitive to block by 4-aminopyridine.

Different calcium dependencies of contractile activity of prostatic and epididymal portions of rat vas deferens

1. The effects of some organic calcium entry blockers and different concentrations of extracellular calcium on electrically-evoked contractions of isolated epididymal and prostatic portions of rat vas deferens were investigated. 2. Both epididymal and prostatic parts of rat vas deferens responded to single pulse or train electrical field stimulation, with twitch contractions of submaximal amplitude. 3. Verapamil showed a biphasic action on the contractions produced by single pulse electrical stimulation. In concentrations < 10(-5) M, it potentiated the responses of both portions, but at higher concentrations, the excitatory action was overcome by a concentration-dependent inhibitory effect. 4. Nifedipine reduced the amplitude of electrically-evoked contractions of both portions in a concentration-dependent manner. The ED50 of nifedipine was 3.6 x 10(-8) M and 2.1 x 10(-6) M in prostatic and epididymal portions, respectively. 5. Dantrolene sodium reduced the amplitude of electrically-evoked contractions of both portions in a concentration-dependent manner. The ED50 of dantrolene was 1.55 x 10(-4) M and 9.1 x 10(-4) M in prostatic and epididymal portions, respectively. 6. Reduction of Ca2+ concentration in medium reduced the amplitude of contractions of both portions significantly. This calcium dependence was more apparent in low frequencies of electrical stimulation.

Effects of nifedipine and ryanodine on adrenergic neurogenic contractions of rat vas deferens: evidence for a pulse-to-pulse change in Ca2+ sources

1. The effects of nifedipine and ryanodine on the adrenergic component of neurogenic contractions of the rat isolated vas deferens were studied in an attempt to identify the sources of Ca2+ mediating the contraction. The tissue was electrically stimulated by single pulses or pairs of widely spaced pulses. The purinergic component of contraction was suppressed by the presence of 300 microM suramin. 2. In Mg(2+)-free medium, nifedipine (0.01-10 microM) reduced the first and, to a greater extent, the second twitch elicited by two pulses 3 s apart. This pattern of inhibition was observed both in the absence of rauwolscine (when twitch 2 was smaller than twitch 1) and in the presence of 0.1 microM rauwolscine (when, due to interruption of prejunctional alpha 2-adrenoceptor-mediated autoinhibition, twitch 2 was of similar height to twitch 1). Nifedipine reduced only twitch 2 but not twitch 1 in medium containing 1.2 mM Mg2+. 3. Single pulses of increasing current strength elicited increasing contraction. Nifedipine reduced contractions by about the same absolute extent at all current strengths, so that the relative contribution of the nifedipine-sensitive component decreased with increasing current strength. 4. When the pulse interval in a pair was increased from 5 to 60 s, the inhibition by nifedipine of the second twitch was most marked at an interval of 5 s and declined as the interval increased. 5. In contrast to nifedipine, 20 microM ryanodine reduced the first twitch of a pair to a greater extent than a second twitch 5 s later, so that twitch 2 became greater than twitch 1. The inhibition by ryanodine of twitch 2 increased with increasing pulse interval.6. In vasa deferentia preincubated with [3H]-noradrenaline, I microM nifedipine and 20 microM ryanodine did not change the electrically evoked overflow of tritium, whereas 10 microM nifedipine increased it.7. It is concluded that, when the sympathetic axons of the vas deferens are stimulated by a single pulse(or the first pulse of a pair) in Mg2+-free medium, both Ca2+ mobilization inside the smooth muscle cells and Ca2+ entry contribute to the ensuing adrenergic contraction. The relative contribution of Ca2+ entry is small at maximal stimulus strength but increases with decreasing stimulus strength. When a second pulse follows the first after an appropriate interval, a switch of Ca2+ sources occurs: intracellular Ca2+mobilization is decreased during twitch 2, whereas Ca2+ entry is increased.

Effects of several calcium channels modulators on the [3H]noradrenaline release and 45Ca influx in the rat vas deferens

1. The effects of nifedipine (1 microM), CdCl2 (0.1 mM) and the Bay K 8644 enantiomers (1 microM) on [3H]noradrenaline release and 45Ca uptake in epididymal and prostatic rat vas deferens were investigated. 2. Nifedipine, CdCl2 and Bay K 8644 optical isomers did not affect the basal tritium release. However, the [3H]noradrenaline release evoked by high potassium (50 mM) from both portions of rat vas deferens was markedly inhibited by CdCl2, scarcely affected by nifedipine and not modified by Bay K 8644 enantiomers. 3. (-)-Bay K 8644 increased the basal and potassium (50 mM) induced 45Ca uptake whereas (+)-Bay K 8644, nifedipine and CdCl2 did not alter the basal 45Ca uptake. However, they strongly inhibited the uptake induced by potassium in both portions of rat vas deferens. 4. These results suggest that the calcium channels (mainly L type) are involved on the contractions in rat vas deferens epididymal and prostatic halves; these channels differ from those present in sympathetic nerve terminals (likely of N Type) which modulates the NA release. 5. This study also shows that Bay K 8644 optical isomers possess opposite effects on the L channels of bisected rat vas deferens smooth muscle.

Characterization of the effects of omega-conotoxin GVIA on the responses of voltage-sensitive calcium channels

1. omega-conotoxin GVIA (omega-CT) caused a potent (IC50 approximately 2nM) but less than maximal (55%) inhibition of [3H]-noradrenaline release from cortical brain slices induced by K+. At 0.1 microM, omega-CT inhibited [3H] gamma-aminobutyric acid (GABA) and [3H]-acetylcholine release by approximately 40%. 2. K+-evoked [3H]-noradrenaline release from cortical brain slices was also characterized with respect to the effects of PN 200-110 (dihydropyridine L-channel antagonist), BAY K8644 (L-channel VSCC agonist), and Cd++ (an inorganic L- and N-channel antagonist). 10 microM Cd++ and 1 microM PN 200-110 inhibited K+-evoked [3H]-noradrenaline release by 52% and 17%, respectively. 10 microM Bay K 8644 enhanced K+-evoked [3H]-noradrenaline release by 22%, and this enhancement was blocked by 1 microM PN 200-110. 3. omega-CT caused a near-maximal inhibition of the electrically evoked twitch responses of the rat vas deferens (IC50 approximately 10 nM) and guinea-pig ileum (IC50 approximately 60 nM), but had no effect on the postjunctional contractile responses of noradrenaline (vas deferens) or carbachol (ileum). At concentrations as high as 1 microM, omega-CT had no effect on the K+-induced contraction of the rat aorta. 4. Neither the equilibrium binding of [3H]-(+)-PN 200-110 nor the allosteric regulation of [3H]-(+)-PN 200-110 binding by tiapamil or diltiazem were altered by omega-CT (0.1 microM). 5. These observations support the notion that the N-type voltage-sensitive calcium channel plays a major role in coupling neuronal excitation with neurotransmitter release.

Effects of the Ca2+ agonists, Bay K 8644 and CGP 28392, on isolated cat cerebral and peripheral arteries

Ca2+-agonists, Bay K 8644 and CGP 28392, produced concentration-dependent contractions in cat middle cerebral arteries. Only Bay K 8644 induced contractile responses in femoral arteries when they were partly depolarized with 15 mM K+. Nifedipine (0.3 microM) caused a reduction of the contractions elicited by low concentrations of both Ca2+ agonists. In femoral arteries, Bay K 8644 (0.1 microM) increased the contractions elicited by K+ concentrations from 25 to 120 mM, whereas CGP 28392 (0.1 microM) only potentiated the response caused by 25 mM K+. The increase of K+-responses induced by Bay K 8644 was blocked by nifedipine (0.1 microM), whereas that produced by CGP 28392 was not only antagonized but the remaining contractions were less than those obtained in the control situation. In both kinds of arteries preincubated with [3H]noradrenaline, nifedipine (50 microM), CGP 28392 (1 microM and 10 microM) and Bay K 8644 (1 microM) neither modified the spontaneous tritium release nor the stimulated tritium secretion elicited by K+ (25, 50 and 75 mM) or electrical stimulation. These data suggest that CGP 28392 and, mainly, Bay K 8644 facilitate the Ca2+ entry into smooth muscle cells, through voltage-dependent Ca2+ channels (that appear to be preactivated in cerebral arteries), whereas those present in the perivascular adrenergic nerve terminals are unaffected. This fact indicates that the channels existent in both vascular structures seem to have different properties.

Verapamil enhances the non-adrenergic twitch response of rat vas deferens

Verapamil (3 X 10(-6)-3 X 10(-5) M) enhanced the twitch contractions of the epididymal and prostatic portions of vas deferens stimulated at 0.1 Hz. This verapamil effect was essentially similar to those of diltiazem, D-600 and Bay K 8644. However, when stimulation at 2 Hz was used verapamil (3 X 10(-5) M) attenuated the contractions of the epididymal portion by half but still augmented those of the prostatic portion. Verapamil enhanced the reserpine- and prazosin-resistant component of the stimulation-induced contractions of both portions of the vas deferens. Yohimbine augmented the twitch response but attenuated the verapamil-augmented response. Verapamil did not augment norepinephrine- or tyramine-induced contractions whereas it augmented ATP-induced contractions of the prostatic portion but not of the epididymal portion. Verapamil increased the stimulation-evoked 3H-efflux from the vas deferens labelled with [3H]norepinephrine. It is suggested that verapamil augments non-adrenergic responses of both portions of the vas deferens by acting as a Ca agonist on the prejunctional site to increase the release of co-transmitter, or by acting on the postjunctional site to enhance the action of the substance released. Its effect in augmenting norepinephrine release is concluded not to contribute to the potentiating action.

Differential effects of Ca antagonists on the noradrenaline release and contraction evoked by nerve stimulation in the presence of 4-aminopyridine

We previously reported that verapamil, nicardipine and diltiazem inhibited both neurotransmitter release and contraction evoked by transmural nerve stimulation (TNS) in the canine saphenous vein. To evaluate whether the three Ca antagonists act on the nerve endings by inhibiting Ca2+ influx, the effects of the three antagonists were studied in the presence of 4-aminopyridine (4-AP) 3 X 10(-4) M on the TNS-evoked tritium overflow and contraction of canine saphenous veins preloaded with [3H]-noradrenaline. 4-AP increased both tritium overflow and contraction evoked by TNS, but did not enhance the contraction induced by exogenous noradrenaline (10 nmol). In the veins pretreated with 4-AP, verapamil (3 X 10(-5) M) and nicardipine (10(-5) M and 3 X 10(-5) M) caused no significant effects on the TNS-evoked tritium overflow, but they still inhibited the contraction. Diltiazem (10(-5) M and 3 X 10(-5) M) significantly inhibited both responses to TNS in the veins pretreated with 4-AP, the effects being nearly equipotent to those in the absence of 4-AP. The (-)-cis isomer of diltiazem (10(-5) M and 3 X 10(-5) M), which is about 100 times less potent than diltiazem in inhibiting Ca2+ influx, inhibited both responses to TNS in the presence of 4-AP to almost the same degree as diltiazem. When 4-AP was added after the Ca antagonists, it reversed the TNS-evoked tritium overflow inhibiting actions of verapamil (3 X 10(-5) M) and nicardipine (3 X 10(-5) M) much more effectively than that of diltiazem (3 X 10(-5) M). Tetracaine (4 X 10(-6) M) significantly inhibited the TNS-evoked tritium overflow and contraction, which were unaffected by 4-AP. Sodium salicylate (10(-2) M) failed to modify the inhibition of TNS-evoked tritium overflow following diltiazem (3 X 10(-5) M), but it enhanced that of tetracaine (4 X 10(-6) M). Verapamil but not diltiazem and nicardipine significantly increased the spontaneous tritium overflow from veins pretreated with 4-AP. The present study together with previous results suggests that diltiazem but not verapamil and nicardipine may inhibit the TNS-evoked neurotransmitter release through an action other than inhibition of Ca2+ influx into the adrenergic nerve endings, allowing an inhibition of the resulting contraction.

Dihydropyridine calcium agonists selectively enhance resistance artery myogenic tone

Upon stretch in normal physiological salt solution (PSS), two dihydropyridine (DHP) Ca2+ agonists (Bay k 8644 and CGP 28392) enhanced the level of stretch-dependent myogenic tone in rabbit ear resistance arteries. This potentiating effect was not blocked by phenoxybenzamine (1 microM) and was Ca2+-dependent. Bay k 8644 was more potent than CGP 28392. These agonists had a smaller potentiating effect on K+ (20 mM)-induced contraction, and had little effect in the presence of high K+ (50-134 mM) PSS. Stretch-dependent myogenic tone is resistant to the inhibitory effect of nimodipine, a selective DHP Ca2+ antagonist which inhibited K+ (50 mM)-induced contraction in the nanomolar range. The potentiating effects of Bay k 8644 and CGP 28392 on myogenic tone were reversed by nimodipine. Our data suggest that DHP Ca2+ agonists selectively enhance myogenic tone by a mechanism that is antagonized by nimodipine, although physiological activation of myogenic tone is nimodipine insensitive, and the calcium related mechanisms of myogenic tone are distinct from those for high K+ depolarization-induced tone.

An investigation of alpha-adrenoceptor responsiveness in the vas deferens of spontaneously hypertensive rats

alpha-Adrenoceptor-mediated responses were investigated in isolated vasa deferentia from spontaneously hypertensive rats (SHR) and normotensive Wistar rats (NWR). There was no significant difference between NWR and SHR in the inhibition of the isometric contraction to single pulse field stimulation by alpha 2-selective agonists in prostatic portions, nor by alpha 2-selective agonists and the alpha 1-selective agonist, amidephrine in epididymal portions in the presence of nifedipine to prevent postjunctional actions of alpha-1-selective agonists. There was no significant difference between NWR and SHR in the potency of amidephrine in causing a postjunctionally mediated potentiation of the isometric contraction to single pulse field stimulation in prostatic portions but the maximum potentiation was significantly reduced in SHR. However, the maximum potentiation of the isometric contraction by the calcium entry facilitator, Bay K 8644, was not significantly different between NWR and SHR. The maximum direct contraction to amidephrine, but not to Bay K 8644, was also significantly reduced in SHR. The irreversible alpha 1-adrenoceptor antagonist, phenoxybenzamine was more potent in SHR than NWR in reducing the maximum potentiation by amidephrine of the stimulation-evoked isometric contraction and in reducing the maximum direct contraction to amidephrine. It is concluded that there is a reduced postjunctional alpha 1-mediated responsiveness in the vas deferens of SHR due probably to a reduction in receptor number.

Differential blocking actions of idazoxan against the inhibitory effects of 6-fluoronoradrenaline and clonidine in the rat vas deferens

The prejunctional inhibitory effects of clonidine and 6-fluoronoradrenaline (6-FNA) have been evaluated in the isolated prostatic segment of the rat vas deferens, against the twitch response evoked by low frequency (0.1 Hz) field stimulation. The inhibitory potency of 6FNA was significantly increased in the presence of cocaine (1 microM) or pargyline (10 microM), but was not modified in the vas deferens from rats pretreated with reserpine when the endogenous levels of noradrenaline (NA) were decreased by 97%. Clonidine was significantly more potent than 6-FNA as an inhibitory agonist, and the potency of clonidine was not modified after cocaine, pargyline or reserpine. The alpha 2-adrenoceptor blocking agent idazoxan, was a competitive antagonist against the inhibitory effects of clonidine under all experimental conditions. In contrast, the only antagonism shown by idazoxan against the inhibitory effects of 6-FNA was in the presence of cocaine (1 microM), and this antagonist effect of idazoxan was not concentration-related. Low concentrations of 6-FNA caused concentration-dependent facilitatory effects on the twitch response, which were significantly greater after treatment with idazoxan (1 microM) in reserpine-treated vas deferens. These facilitatory effects of 6-FNA were always observed in the presence of prazosin (300 nM) and also after treatment of the preparations with phenoxybenzamine (10 microM), a concentration which abolished the inhibitory actions of both clonidine and 6-FNA. The facilitatory effects on the twitch response induced by low concentrations of 6-FNA are therefore unlikely to be due to either alpha 1- or alpha 2-adrenoceptor stimulation. In conclusion, the failure of idazoxan to block the inhibitory effects of 6-FNA, while exerting a potent competitive antagonism of clonidine-induced inhibitory effects, supports the proposal that alpha 2-adrenoceptors may in fact be subdivided into two subclasses, involving imidazoline and phenylethylamine recognition sites.

Inhibitory effect of adenosine on electrically evoked contractions in the rat vas deferens: calcium dependence

The inhibitory effect of adenosine on the electrically evoked contractions in the rat prostatic vas deferens was markedly dependent on the calcium concentration in the medium, with adenosine being 20 times more potent at lower calcium levels (1.26 mM vs 3.6 mM). Similar calcium dependence of adenosine inhibition was observed in the presence of a potent nucleoside uptake inhibitor, nitrobenzylthioguanosine (NBTGR), suggesting that this calcium dependence of adenosine inhibition was not secondary to an alteration in adenosine uptake. Furthermore, Bay K8644(methyl-1,4-dihydro-2,6- dimethyl-3-nitro-4-(2-trifluoromethylphenyl)pyridine-5-carboxylate), a dihydropyridine 'calcium entry activator', reduced the potency of adenosine in inhibiting the electrically evoked contractions in a calcium-dependent manner. These results support the notion that adenosine might exert its inhibition on neurotransmission in the prostatic rat vas deferens by decreasing transmembrane calcium currents into the nerve terminals and/or the smooth muscles.

Further examination of the inhibitory actions of alpha 1-adrenoceptor agonists in rat vas deferens

The inhibitory actions of alpha 1-adrenoceptor agonists were examined in the isolated bisected vas deferens of the rat. The calcium entry facilitator Bay K 8644 markedly potentiated the isometric contraction to a single stimulus pulse in epididymal portions of rat vas deferens: subsequent amidephrine produced an inhibition which was antagonised by the alpha 1-adrenoceptor antagonist, prazosin, but not by the alpha 2-adrenoceptor antagonist, yohimbine. The alpha 1-adrenoceptor agonists amidephrine and cirazoline failed to inhibit the transmitter overflow to trains of pulses at a frequency of 2 Hz in epididymal portions, but also failed to abolish the nifedipine-resistant adrenergic contraction to trains of pulses at 2 Hz in epididymal portions. It is concluded that alpha 1-adrenoceptor agonists have inhibitory effects which may be by action at presynaptic alpha 1-adrenoceptors.

Further examination of the effects of ageing on the adrenoceptor responsiveness of the rat vas deferens

Age-related changes in presynaptic alpha 2-adrenoceptors were investigated in prostatic and epididymal portions of vasa deferentia from young adult (2-3 months) and old (24-29 months) Sprague-Dawley rats, using the alpha 2-selective agonists xylazine and clonidine. In prostatic portions the inhibitory effects of clonidine against the isometric contractions to single pulse field stimulation were complicated by a postsynaptic action in old animals, but in epididymal portions in the presence of nifedipine both xylazine and clonidine were 3 times less potent in old rats. However, there were no significant differences between young and old in the potency of xylazine at inhibiting the overflow of tritium or the isometric contraction evoked by 5 Hz stimulation for 3 min in tissues pre-incubated with [3H]noradrenaline. It is suggested that there is reduced responsiveness of presynaptic alpha 2-adrenoceptors in the vas deferens of old rats, but that this can only be demonstrated using a sensitive measure of the presynaptic potency of agonists: the isometric contraction to a single stimulus.