Biomedical subjects

R P Markus

Publications and source records attributed to R P Markus.

At least 37 records · Page 2Linked to original sources

PubMed1996

Are imidazoline receptors involved in sympathetic neurotransmission in rat vas deferens?

1. An involvement of imidazoline receptors in the modulation of neurotransmitter release was investigated in the prostatic portion of the rat vas deferens stimulated transmurally at 0.2 Hz or by single pulses. 2. Idaxozan and yohimbine induced a concentration-dependent potentiation of the contractile response to 0.2-Hz transmural stimulation in the epididymal and prostatic portion of the vas. 3. After reserpine treatment, idazoxan, but not yohimbine, still potentiated the contractile response, suggesting a possible involvement of imidazoline receptors. 4. Clonidine and rilmenidine, agonists with different affinities to alpha 2-adrenoceptors and imidazoline receptors, inhibited with the same potency the contractile responses to a single pulse transmural stimulation. 5. Yohimbine (a selective alpha 2-adrenoceptor antagonist) antagonized the inhibitory concentration effect curve to rilmenidine in a competitive manner. pA2 values for idaxozan (an antagonist to alpha 2-adrenoceptors and imidazoline receptors) were not different when noradrenaline or rilmenidine were used as agonists. Phenoxybenzamine blocked the effect of both agonists. 6. Thus, the potency relationship of agonists, as well as the effect of the antagonists, did not favor the hypothesis that imidazoline receptors are involved in the idazoxan-potentiating effect in the rat vas deferens.

Adrenergic alpha-2 Receptor Agonists↗

PubMed1996

Melatonin modulation of presynaptic nicotinic acetylcholine receptors in the rat vas deferens.

Melatonin, the pineal hormone produced during the dark phase of the light-dark cycle, modulates neuronal acetylcholine receptors located presynaptically on the sympathetic nerve terminals of the rat vas deferens. The pD2 values for nicotine were significantly higher at night (4.20 +/- 0.01) than in the afternoon (3.80 +/- 0.07). Exogenous melatonin shifted the concentration-response curves for nicotine to the left, mimicking the effect of darkness. Melatonin modifies both the displacement and the saturation curves of [3H](-)nicotine binding. In membranes from animals killed at 15:00 h, the binding of [3H](-)nicotine (5-6 nM) was first potentiated and then inhibited by sequential concentrations of (-)nicotine. Higher concentrations of [3H](-)nicotine (50-60 nM) were displaced by all concentrations of nonlabeled ligand. However, when membranes from tissues exposed to melatonin (exogenous or endogenous) were tested, the lower [3H](-)nicotine concentration was displaced progressively by increasing concentrations of nonlabeled ligand. Equilibrium binding studies show a single class of high-affinity nicotinic binding sites with an apparent Kd value of 16 nM and an average maximal number of binding sites of 66 fmol mg-1 protein when animals were killed at the afternoon. Melatonin, although it did not change the properties of high-affinity binding sites, induced the appearance of a second population of lower apparent affinity (Kd = 36.7 nM; Bmax = 185.4 fmol/mg). Melatonin does not modify the functional response and the displacement of [3H](-)nicotine by dimethylphenylpiperazinium. The data suggest that nicotinic neuronal acetylcholine receptors stimulated by dimethylphenylpiperazinium do not change between the light and dark phases. Rather, the higher sensitivity to nicotine in prostatic portions incubated with exogenous melatonin, and in organs from animals killed at night, after the rise of endogenous melatonin, is probably due to the appearance of low-affinity neuronal nicotinic ACh binding sites.

Acetylcholine↗

PubMed1994

Modulation of sympathetic neurotransmission by melatonin.

Melatonin is of considerable interest for its regulatory influence on a variety of physiological processes including biological rhythms and neuroendocrine functions. We showed that melatonin potentiates sympathetic neurotransmission in the prostatic portion of the rat vas deferens, by increasing contractions in response to noradrenaline and ATP released by acetylcholine stimulation of presynaptic nicotinic receptors. Melatonin in vitro (100 pg/ml; for 4 h) increased the maximal acetylcholine-induced contraction only when the hypogastric ganglion was present, and this effect was blocked by cycloheximide (100 micrograms/ml). Melatonin also modulated the sympathetic trophic influence on smooth muscle, since it reduced [35S]methionine incorporation into the vas deferens in the hypogastric ganglion-vas deferens preparation. Thus, it is suggested that the regulation of protein synthesis might be one of the mechanisms whereby melatonin modulates endogenous rhythms and synchronizes them to the environmental light cycle.

Acetylcholine↗

PubMed1994

Presence of P2-purinoceptors in the rat pineal gland.

1. The effects of noradrenaline, ATP, adenylyl-imidodiphosphate (AMP-PNP), adenosine, alpha,beta-methylene-ATP and the P2-purinoceptor antagonist, suramin on N'-acetyl-5-hydroxytryptamine production were studied in cultured denervated rat pineal glands. 2. Noradrenaline (3 nM-1 microM) increased N'-acetyl-5-hydroxytryptamine production as measured both in the gland and the culture medium. 3. In noradrenaline (10 nM)-stimulated pineal glands, ATP (0.03 nM-1 mM) or AMP-PNP (0.1 microM-1 mM) increased N'-acetyl-5-hydroxytryptamine production in a concentration-dependent manner. 4. Alpha,beta-Methylene-ATP at the concentration of 0.1 mM, but not 3 microM, attenuated the enhancement by ATP (0.1 mM) of noradrenaline (10 nM)-induced N'-acetyl-5-hydroxytryptamine production. 5. Suramin (0.1 mM) blocked the potentiating effect of ATP (0.1 mM), but not the potentiating effect of adenosine (0.1 mM) in glands incubated with noradrenaline (10 nM). 6. These findings suggest that the rat pineal gland possesses P2-purinoceptors which when stimulated potentiate the effect of noradrenaline but do not, by themselves, induce an increase in N'-acetyl-5-hydroxytryptamine production.

Adenosine↗

PubMed1993

Circadian variations of superoxide dismutase activity in the rat pineal gland.

The 24 h profile of the activity of the antioxidant enzyme superoxide dismutase (SOD) in the pineal gland of rats was studied. Rhythmic analysis showed a significant 24 h rhythm with an amplitude of oscillation of 25% of the 24 h mean value, that was 100.34 +/- 1.6 U SOD (nitrite). An ultradian rhythm of 9 h was also detected. The diurnal profile of superoxide dismutase activity is discussed in relation to the oxidative metabolism of the pineal gland.

Activity Cycles↗

PubMed1993

Age-related changes in norepinephrine release and its modulation by presynaptic alpha-2 adrenoceptors in the rat vas deferens.

Our objective was to extend the study of age-related changes in the modulation of norepinephrine release by presynaptic alpha-2 adrenoceptors. The experimental model utilized was the epididymal and prostatic portion of the rat vas deferens, because the two portions differ in age-related changes and pattern of sympathetic neurotransmission. The functional influence of presynaptic alpha-2 adrenoceptors was evaluated by quantifying the potentiating effect of yohimbine on the contraction induced by transmural stimulation. Yohimbine-induced potentiation was much more pronounced in the epididymal portion of young rats and was reduced by aging only in this portion. Potassium-induced release of 3H-norepinephrine was significantly higher in the epididymal than in the prostatic portion, and an age-related reduction in 3H-norepinephrine release occurred only in the epididymal portion. However, aging did not change the inhibitory effect of clonidine on the contraction induced by transmural stimulation (0.2 Hz or single pulse) in either portion. Thus the age-related decline in alpha-2 feedback regulation is probably not directly related to changes in prejunctional alpha-2 adrenoceptor activity but may be associated with age-related changes in the amount of norepinephrine that stimulates this receptor. However, the lower overflow of neurotransmitter in the epididymal portion did not translate into age-related changes in the contractile responses to transmural stimulation (2-20 Hz). In the prostatic portion, aging increased the functional neurotransmission, partially because of a reduction in neuronal norepinephrine uptake. The results suggest that age-related changes are specific for each portion and may be associated with the pattern of neurotransmission of the tissue.

Aging↗

PubMed1993

Age-related changes in melatonin modulation of sympathetic neurotransmission.

An age-related reduction in the maximal acetylcholine (ACh)-induced contraction in the prostatic portion of rat vas deferens during the day was observed. This contraction is due to the release of norepinephrine and adenosine triphosphate from sympathetic nerve terminals. Male Wistar rats (4 and 24 months-old) were housed on a light/dark cycle (12:00 h/12:00 h, lights on at 6:00). The diurnal variation of ACh-induced contraction was determined on animals sacrificed every 3 h during the day. Aging reduced the maximal contraction and shortened the length of the nocturnal response. In both young and old rats, the response to ACh increased at 21:00, but it remained elevated until 12:00 in young rats and until 24:00 in old rats. Both nocturnal administration of melatonin for 2 consecutive days (9 mg/kg/day, s.c.) and melatonin incubation (100 pg/ml, 4 h) of the vas deferens from old rats sacrificed at 15:00 h significantly potentiated the ACh-induced contraction. However, this potentiation was smaller than that observed in young rats. The results presented here show the loss of maximal contractile response to ACh with age and a qualitative change in the rhythm characteristics of this phenomenon. Moreover, the age-related decrease in the ACh-induced contraction in the prostatic portion of the rat vas deferens is dependent not only on a reduction in the plasma nocturnal increase of melatonin concentration but also on a lower responsiveness of short sympathetic neurons to melatonin.

Acetylcholine↗

PubMed1992

Effect of ageing on the number of neuronal noradrenaline uptake sites in the rat vas deferens.

Previous work has shown an age-related reduction in neuronal uptake of noradrenaline in the prostatic, but not in the epididymal portion of the rat vas deferens. In the present paper, the influence of ageing on the number of [3H]desipramine binding sites and on the effect of lithium on neuronal [3H]noradrenaline uptake were studied in the prostatic and epididymal portions of vasa deferentia from 4- and 20-month-old rats. The affinity for [3H]desipramine (Kd values) in the epididymal and prostatic portions did not change with age. However, ageing reduced the maximal number of [3H]desipramine binding sites (Bmax values) in the prostatic, but not in the epididymal portion. Lithium potentiated neuronal [3H]noradrenaline uptake only in the prostatic portion and this potentiation was not changed by ageing. The results showed differences in neuronal noradrenaline uptake between the two portions of the vas deferens. Furthermore, the data suggest that the age-related reduction in neuronal uptake in the prostatic portion is due to a reduction in the number of neuronal uptake sites for noradrenaline.

Aging↗

PubMed1991

Diurnal variation of the rat vas deferens contraction induced by stimulation of presynaptic nicotinic receptors and pineal function.

A rhythmic variation of maximal contraction induced by acetylcholine in the prostatic portion of rat vas deferens was tested. This contraction is due to the release of norepinephrine and ATP from sympathetic nerve terminals. Male Wistar rats (4 months old) were housed on a light/dark cycle (12 hr/12 hr, lights on at 6:00 A.M.). The diurnal variation of acetylcholine-induced contraction was determined on animals sacrificed every 3 hr during the day. The maximal contractile response shows a circadian (24:00 hr) and an ultradian (12:20 hr) rhythm. Otherwise, the sensitivity to acetylcholine (pD2 values) and the maximal contraction or pD2 values to norepinephrine, ATP and K+ did not change throughout the day. The blocking effect of hexamethonium on the contraction induced by field stimulation was higher at 9:00 P.M. than at 3:00 P.M., indicating a diurnal variation of the effect of endogenous released acetylcholine. When melatonin released by the pineal gland is suppressed by constant illumination or superior cervical ganglionectomy, the circadian rhythm was abolished and the period of the ultradian rhythm changed to 6:30 hr. The acetylcholine-induced contraction of vasa deferentia from animals sacrificed at 3:00 P.M. and incubated "in vitro" with melatonin (100 pg/ml) increased reaching nocturnal values. In conclusion, it seems that a functional pineal gland, most probably through the synthesis and release of melatonin, is necessary for expression (circadian) and modulation (ultradian) of the rhythmicity in the maximal acetylcholine-induced contraction in the prostatic portion of the rat vas deferens.

Acetylcholine↗

PubMed1990

Age-related changes in neuronal uptake of noradrenaline.

Age-related changes in the neuronal uptake of noradrenaline were studied in the vas deferens of 4-, 12- and 20-months old rats by determining the potentiating effect of cocaine on concentration-effect curves to noradrenaline and by evaluating the kinetic parameters (Vmax and apparent Km) of neuronal 3H-noradrenaline uptake. Since the neurotransmitters released by nerve terminals and the density of noradrenergic innervation are different in the epididymal and prostatic portion of the vas, the experiments were carried out in these two portions of the organ. In the epididymal portion, the ratio between EC50 for noradrenaline in the presence and in the absence of cocaine was reduced in tissues of middle-aged (12-months) and old (20-months) animals when compared with young (4-months) animals. The pD2 values for noradrenaline in the absence of cocaine and the kinetic parameters for neuronal 3H-noradrenaline uptake were not modified by ageing. Thus, in the epididymal portion there was no age-related change in neuronal uptake. The age-related decrease in the cocaine-induced supersensitivity to noradrenaline was due to an age-related decrease in the pD2 value for noradrenaline observed in the presence of cocaine, suggesting postsynaptic changes. In the prostatic portion, the ratio between EC50 for noradrenaline in the presence and in the absence of cocaine was also reduced in tissues of old animals (20-months) when compared with middle-aged and young animals. The pD2 value for noradrenaline in the absence of cocaine obtained in the prostatic portion of 20-months old rats was higher than that in the younger groups (4- and 12-months).(ABSTRACT TRUNCATED AT 250 WORDS)

Aging↗

PubMed1990

Neuronal uptake of noradrenaline in the rat isolated trachea: effect of ageing.

1. The neuronal uptake of 3H-noradrenaline in isolated trachea strips of the rat was analysed from cocaine-resistant 2 min accumulation of 3H-noradrenaline from reserpine pre-treated animals. 2. Neuronal uptake of 3H-noradrenaline was demonstrated to exist in isolated trachea of rats at different ages (4, 12 and 20 months old). 3. The determination of kinetic parameter values (Vmax and apparent Km) of neuronal 3H-noradrenaline uptake indicated that this transport process was not modified by ageing.

Aging↗

PubMed1990

Presynaptic nicotinic receptors involved in release of noradrenaline and ATP from the prostatic portion of the rat vas deferens.

A differential response to cholinomimetic agonists in epididymal and prostatic portions of rat vas deferens was characterized. The prostatic portion was less sensitive to acetylcholine and carbachol than the epididymal portion. The contraction induced by cholinomimetic agonists was inhibited in the epididymal portion by atropine (1.0-3.0 nM) and in the prostatic portion by hexamethonium (0.1 mM). The contractile response of the prostatic portion to exogenous acetylcholine was not inhibited by textrodotoxin (1.0 microM) but was attenuated by reserpine treatment (10 mg.kg-1 i.p. 24 h) and by prazosin or alpha, beta-methylene ATP. A combination of an alpha-1-adrenoceptor antagonist (prazosin) and P2 purinoceptor desensitization with alpha, beta-methylene ATP abolished the contractile response of the prostatic portion. The contraction induced by repetitive field stimulation of the prostatic portion was attenuated by hexamethonium whereas the response to a single stimulus was not modified. The data suggest that cholinomimetic drugs activate both nicotinic receptors located in nerve terminals of the prostatic portion and muscarinic receptors located in the smooth muscle cells of the epididymal portion, and that stimulation of nicotinic receptors induces the release of noradrenaline and ATP.

Acetylcholine↗

PubMed1989

Do differences in innervation result in different post-synaptic responses to exogenous agonists?

1. The pharmacological reactivity of the epididymal and prostatic portions of the rat vas deferens to BaCl2, phenylephrine and carbachol were recorded by isometric and isotonic technique. 2. The maximum response induced by the three agonists were similar at the epidiymal end, while at the prostatic portion phenylephrine produced a response 80% lower than that of barium and carbachol. 3. The pD2 value to agonists and the sensitivity to calcium channel blockers were lower at the prostatic end. 4. The data suggest that not only the pharmacological reactivity of the prostatic and epididymal portions differs, but also that the activity of the prostatic portion is much more reduced to alpha 1-agonists.

Animals↗

PubMed1988

Age-related changes in neuronal uptake of catecholamines.

Age-related changes in the neuronal uptake of labelled noradrenaline were analyzed in transversally sectioned portions of 4-, 12- and 20-month old rat vas deferens. Uptake was a saturable process apparently following the Michaelis-Menten equation. By determining the values of Km and Vmax, it was possible to conclude that neuronal uptake does not change with age in the epididymal portion of the vas deferens and is reduced in the prostatic portion of 20-month old rats.

Aging↗

PubMed1987

Influence of castration on the membrane reactivity of the guinea-pig vas deferens.

The guinea-pig vas deferens is a quiescent muscle which after castration undergoes atrophy and shows spontaneous contractions preceded by membrane spike activity. The influence of castration on the spontaneous release of neurotransmitters and on the internal concentration of sodium and potassium ions was studied. Utilizing the microelectrode technique it was shown that castration induces a partial depolarization (10 mV) of the cell membrane, but did not change the frequency of spontaneous excitatory junction potentials (SEJPs) of guinea-pig vas deferens. However, the time-course and the amplitude of the SEJPs were increased after castration, probably because of changes in membrane properties related to organ atrophy. Castration probably promotes a change in the ionic permeability of the smooth muscle fibre, since the ratio pNa/pK was twice that of control muscles.

Animals↗

PubMed1987

Effects of sodium and calcium concentration on the barium chloride-induced electrical and contractile responses of the guinea pig vas deferens.

1. The mechanism of 10 mM barium chloride-induced electrical and contractile responses of the guinea pig vas deferens was investigated using calcium channel blockers and by modifying the Na+ and Ca2+ concentrations of the nutritional solution. Isometric contraction and membrane depolarization were measured simultaneously by the sucrose-gap technique. 2. In the absence of added Ca2+ there was a decay of the contractile but not of the depolarizing effect, which was independent of the frequency of 10 mM barium chloride administration. However, both barium-induced contraction and depolarization were reduced and the former more rapidly in a Ca2+-free solution (0.5 mM EGTA). 3. In low-Na+ solution (16 mM) there was a 3-fold increase in depolarization and no change in contraction, but in a low-Na+ Ca2+-free solution (0.5 mM EGTA), only contraction was reduced. 4. Nifedipine blocked both barium-induced contraction and depolarization in a dose-dependent manner when the organs were bathed in regular nutrient solution. In contrast, in low-Na+ solution only 50% of the depolarization was blocked by 10(-6) M nifedipine, which completely inhibited contraction. Both barium-induced contraction and depolarization in low-Na+ Ca2+-free solution were blocked by 10(-3) M LaCl3. 5. These data indicate that in the presence of calcium, barium mobilizes extracellular calcium through voltage-dependent channels and the contraction is due to the entry of calcium. In contrast, in the absence of added Ca2+ or in Ca2+-free solutions (0.5 mM EGTA), barium enters through the voltage-dependent channel and the contraction is reduced because the intracellular stores of calcium are limited.

Animals↗

PubMed1986

The time-course of contractions induced by acetylcholine and barium in the vas deferens of normal and castrated guinea-pigs.

1. The influence of castration on contraction of the guinea pig vas deferens induced by acetylcholine was investigated regarding facilitation of endogenous neurotransmitter release, changes in post-synaptic events and modification in acetylcholine metabolization. 2. Castration prolonged the duration of the isometric contraction of the vas deferens induced by acetylcholine (3 mM) but not the duration of that induced by barium chloride (10 mM). 3. Reserpinization (0.5 mg/kg) of normal and castrated guinea pigs did not change the time-course of acetylcholine-induced contraction in the vas deferens. 4. 4-Aminopyridine (0.05 mM) prolonged the contraction induced by acetylcholine and barium chloride. This effect was blocked by reserpine pretreatment, indicating that in the presence of 4-AP, both acetylcholine and barium release endogenous noradrenaline. Since there was no difference between the effects of 4-AP on organs from normal and castrated animals, prolongation of the acetylcholine-induced contraction is not related to an increase in neutransmitter release. 5. Total cholinesterase and acetylcholinesterase activities were reduced after castration and the time-course of carbachol contraction was not changed. Thus, this decrease in enzyme activity appears to play a relevant role in the prolongation of acetylcholine-induced contraction.

Acetylcholine↗

PubMed1985

Age-related changes in the reactivity of the rat jejunum to cholinoceptor agonists.

The contractile effects of barium, potassium, acetylcholine and acetyl-beta-methylcholine were studied in jejunum of 4, 12 and 20 month-old rats. The effects of age on the sensitivity and responsiveness of the jejunum to the agonists were examined using cumulative dose administration and measuring the pD2. The maximum contraction produced by the agonists and the sensitivity to barium and potassium were not modified by aging. In the middle-aged animals, the jejunum sensitivity to acetylcholine decreased while the sensitivity to acetyl-beta-methylcholine was not statistically different from that of 4 month-old rats. Since at this age (12 months) the acetylcholinesterase activity increased, the reduction in the sensitivity to acetylcholine could have been due to the increase in the activity of the metabolizing enzyme. Otherwise, in the jejunum from the older animals the sensitivity for both cholinoceptor agonists, acetylcholine and acetyl-beta-methylcholine, increased 5-fold when compared with the middle-aged animals. However, compared with the jejunum of 4 month-old rats the sensitivity to acetylcholine, to 20 month-old animals, was not altered while the sensitivity to acetyl-beta-methylcholine was increased. At this age (20 months), the jejunum acetylcholinesterase activity was increased when compared with that of 4 month-old rats and was not modified when compared with that of 12 month-old rats. The change in supersensitivity to cholinoceptor agonists could not have been due to an alteration in muscarinic receptor affinity, since the pA2 for atropine was not modified with increasing age. These results suggest that after an increase in acetylcholinesterase activity around month 12, the jejunum develops an adaptative supersensitivity to cholinoceptor agonists.

Acetylcholine↗