Effects of ranitidine on the enzyme cholinesterase and the rat anococcygeus muscle

Ranitidine in lower doses, (100 ng and 1 g) accelerated the rate of reaction of the enzyme acetylcholinesterase with the substrate acetylthiocholine. However, in higher doses (10 g and 100 g) it inhibited the enzyme activity. In rat anococcygeus muscle preparation, the responses to acetylcholine were significantly inhibited in lower doses whereas in higher doses there was a dose-dependent potentiation of the responses to acetylcholine by ranitidine. The responses to carbachol were however, not affected by ranitidine in the same preparation. Our data suggest cholinomimetic as well as cholinolytic activity of ranitidine.     

Polymerization of myosin on activation of rat anococcygeus smooth muscle

The in vivo state of assembly ofmyosin in vertebrate smooth muscle is controversial. In vitrostudies on purified smooth muscle myosin show that it ismonomeric (10S) under relaxing conditions and filamentous undercontraction conditions. Electron microscopic and antibodylabelling studies of intact smooth muscles, on the other hand,suggest that myosin is filamentous in the relaxed as well as thecontracting state and that 10S myosin occurs only in traceamounts. However, birefringence, conventional EM and X-raydiffraction evidence suggests that in certain smooth muscles invivo (e.g. rat anococcygeus), while myosin filaments exist in therelaxed state, their number increases on contraction. Here, wehave used low temperature electron microscopic techniques (rapidfreezing followed by freeze-substitution), which preserve labilecomponents in close to their in vivo state, to detect any changein filament number on contraction. The results from ratanococcygeus have been compared with those from guinea pig taeniacoli, in which other techniques have revealed no change infilament number. In the anococcygeus, we find evidence for a 23%increase in filament density in transverse sections ofcontracting muscle compared with relaxed muscle. In the taeniacoli we find no change. These results are in qualitativeagreement with earlier findings. They provide evidence forpolymerization of myosin in contracting rat anococcygeus, andsuggest that this process is subtle and occurs only in somesmooth muscles     

Membrane properties of the smooth muscle cells of the rat anococcygeus muscle.

1. The membrane properties of the rat anococcygeus muscle, during rest and activity, were investigated with micro-electrodes and partition stimulation. 2. Intercellular current spread occurred within the muscle and the mean length constant was 2-7 mm. The membrane showed rectification to depolarizing pulses. 3. The mean resting potential was --62-1 mV and the input resistance was 23-0 MOMEGA. Stimulation of intramural nerves produced depolarization to --21 mV and a 10% reduction in input resistance. Displacement of the membrane potential indicated that the transmembrane potential at the peak of the response was independent of the membrane potential. 4. Noradrenaline also produced depolarization and this was accompanied by a decrease in membrane resistance as indicated by a reduction in amplitude of the electrotonic potential. 5. It was concluded that the muscle possesses cable properties and that the action of the transmitter, noradrenaline, is to increase membrane permeability so that the membrane potential moves towards an equilibrium potential.     

The effects of intrathecally applied noradrenaline and 5-hydroxytryptamine on spinal nocifensive reflexes and the rostral transmission of noxious information to the thalamus in the rat

The effects of intrathecally applied noradrenaline (NA) and 5-hydroxytryptamine (5-HT) on a spinal nocifensive reflex and nociceptive responses recorded from rat ventrobasal thalamus have been compared. A dose of 15 nmol NA increased the tail flick latency (TFL) for approximately 120 min (n = 12) in rats lightly anaesthetised with Saffan. A dose of 260 nmol 5-HT increased the TFL for approximately 21 min (n = 7). In rats anaesthetised with urethane, 15 nmol NA produced a reversible reduction in the response of 15 ventrobasal thalamic units to noxious stimulation lasting approximately 36 min (n = 15). A dose of 260 nmol 5-HT reduced thalamic nociceptive responses for approximately 25 min (n = 12). This suggests that spinal interneurones subserving the tail flick reflex are more sensitive to NA than spinal neurones involved in the transmission of noxious information supraspinally. In contrast, intrathecally applied 5-HT is equipotent in its action on both groups of neurones involved in nociceptive mechanisms.     

The effect of temperature elevation on the cerebrovascular response to noradrenaline and 5-hydroxytryptamine

Fever frequently complicates stroke and subarachnoid haemorrhage. A transient rise in transmitter monoamine levels of plasma and cerebrospinal fluid occurs in these diseases. The present study demonstrates an enhanced vasoconstrictor response of cerebral vessels to noradrenaline—but not 5-hydroxytryptamine—following a rise in temperature. The augmented response is more likely due to an impaired inactivation (re-uptake) of the amine than to an altered sensitivity of the post-synaptic α-adrenergic receptor, since it could be reproduced by pretreatment with cocaine. The finding indicates that it may be important to combat fever in these patients.     

Contractile effects of noradrenaline and 5-hydroxytryptamine in human hand veins: a pharmacological receptor characterization

The postjunctional receptors mediating contractile responses to noradrenaline (NA) and 5-hydroxytryptamine (5-HT) were characterized in ring segments of human hand veins by using subtype selective agonists and antagonists. The mechanical characteristics of the preparations were also examined by length-tension measurements. The length-active wall tension curve was bell-shaped and reached a maximum at a length corresponding to a passive distending pressure of approximately 14 mmHg. (-)-Phenylephrine consistently contracted the veins and was 24 times less potent than (+/-)-NA whereas clonidine produced a contraction in only two out of 11 vessel segments. Neither prazosin nor rauwolscine competitively inhibited the contractile response to NA, and large inter-individual differences were found in the degree of inhibition produced by the antagonists. However, application of both prazosin and rauwolscine almost abolished the NA-induced contraction. Ketanserin and methergoline inhibited the contractile response to 5-HT; the former in an apparently competitive manner with a pA2 value of 8.94, whereas the latter substantially reduced the maximum 5-HT response. It is suggested that NA elicits contraction in human hand veins by acting at a mixed population of alpha 1- and alpha 2-adrenoreceptors. The contractile response to 5-HT, on the other hand, appears to be mediated predominantly by 5-HT2 receptors.     

Some effects of 5-hydroxytryptamine, dopamine and noradrenaline on neurones in the submucous plexus of guinea-pig small intestine.

1. Responses to the iontophoretic application of 5-hydroxytryptamine (5-HT), dopamine and noradrenaline were examined in neurones of the submucous plexus of guinea-pig small intestine. 2. Every neurone was excited by 5-HT. 3. In a proportion of cells, dopamine or noradrenaline caused an increase in membrane potential. This response was only observed in cells which received in inhibitory innervation. The responses closely resembled inhibitory synaptic potentials evoked by transmural stimulation. 4. Both inhibitory synaptic potentials and inhibitory responses to dopamine and noradrenaline were blocked by methysergide. 5. It seems possible that these two catecholamines may interact with similar receptors to those activated by inhibitory transmitter.     

The effect of palytoxin on neuromuscular junctions in the anococcygeus muscle of the rat

Palytoxin, a highly toxic natural product isolated from zoanthids of the genus Palythoa, is accumulated by a wide range of fishes and marine invertebrates used as food in the Indo-Pacific. It is responsible for many incidents of human morbidity and mortality. The toxin is a potent smooth muscle spasmogen. The cause of the contraction of smooth muscle is unclear, but recent work strongly suggests that it is primarily initiated by the release of neurotransmitters from the motor innervation of the smooth muscle. We show here that palytoxin caused the swelling of the muscle cells and some internal organelles of the anococcygeus muscle of the rat, but no substantial structural damage to the tissue. Axons and Schwann cells were also swollen but the most dramatic feature was the depletion of synaptic vesicles from putative release sites in the axons. Some axons were physically damaged following exposure to the toxin, but this was relatively uncommon (<10% of all axons studied). In the majority of axons there was no damage to nerve terminal membranes, but there was damage to mitochondria. The depletion of vesicles involved all types – clear, dense-cored, large and small. Our observations and pharmacological data gathered elsewhere, provide a neuropathological basis for the spasmogenic activity of palytoxin.     

An excitatory action of adrenaline, noradrenaline, and 5-hydroxytryptamine on the spinal cord

1. Fasciculation was produced in the trapezius and sternomastoid muscles of anaesthetized cats by adrenaline, noradrenaline and 5-hydroxytryptamine (5-HT) put into the cisterna magna. Isoprenaline, similarly applied, was ineffective.

2. Each of the three active amines was without effect when applied for a second time after the fasciculation in response to the first application had passed off, but 5-HT was effective after adrenaline and, similarly, adrenaline after 5-HT.

3. Fasciculation produced by adrenaline, but not by 5-HT, was inhibited by ergotamine or phenoxybenzamine, given intravenously.

4. In producing fasciculation, the adrenaline appeared to be acting on or through the lateral aspect of the upper cervical cord, about the line of emergence of the roots of the spinal accessory nerve, mainly in C1 and C2.

5. The electromyograms of the fasciculation due to adrenaline and 5-HT showed intermittent bursts of activity. After adrenaline, the bursts consisted of fewer spikes than after 5-HT. The intervals between consecutive spikes were 3·75-5 msec after either amine, and the bursts occurred irregularly, at frequencies between 7 and 12/sec.

6. It is suggested that adrenaline and 5-HT have excitatory actions on the dendrites or somata of the spinal accessory motor neurones, and the possible role of these amines as synaptic transmitters in the spinal nucleus of the accessory nerve is discussed.

     

A morphological study of the effects of 5-hydroxytryptamine and indomethacin on rat mesenteric venules.

A method is described for preparing venules of the rat mesentery for electron microscopy after the application of 5-hydroxytryptamine (5-HT) and pretreatment with indomethacin. Local application of 5-HT caused the leakage of colloidal carbon and the emigration of leucocytes into the venule wall. 5-HT also caused endothelial cells to bulge and their nuclei to contort. It increased the number of protrusions on both the luminal and abluminal surfaces of the endothelium and increased the width of the subendothelial space, and the degree of vesiculation in the endothelial cells. Systemic treatment with indomethacin significantly decreased the amount of carbon passing through the endothelium after the local application of 5-HT, but enhanced some of the other effects of 5-HT. Thus it increased the bulging of endothelial cells and contortion of their nuclei, and further increased the number of surface protrusions and the subendothelial space. It had no effect on the emigration of leucocytes resulting from the application of 5-HT.     

The action of 5-hydroxytryptamine on Mytilus smooth muscle

1. In the nerve—muscle preparation, where catch was characteristically minimal, 5-hydroxytryptamine (5-HT) had no effect on resting membrane potential, junction potentials, spikes or contraction.

2. In muscle bundles, where catch was prominent, 5-HT did not change membrane potentials, but prolonged junction potentials and lowered the threshold for spike discharge and contraction.

3. In muscle bundles, exposed to high concentrations of 5-HT, depolarization evoked repetitive spikes, while in low 5-HT, spikes were seldom fired even with much greater depolarization.

4. In muscle bundles, the effective membrane resistance, R_eff., decreased from 45-60 to 23-35 MΩ as 5-HT concentration was increased.

5. It is suggested that 5-HT may facilitate spike discharge by lowering the internal free Ca²⁺ concentration.

     

Secretory effects of 5-hydroxytryptamine following neonatal sympathectomy in rat parotid gland

Unilateral sympathetic denervation of rat parotid glands was performed within 4 h after birth. Nine weeks later the glands were used for in-vitro studies of amylase secretion, and ⁸⁶Rb+ was used as a marker for potassium efflux. The non-denervated contralateral glands served as controls. The tissue concentrations of 5-hydroxytryptamine and its metabolite 5-hydroxyindole acetic acid were also measured. 5-Hydroxytryptamine caused a significant dose-dependent increase in amylase secretion, which was inhibited by methysergide. There was no difference between controls and denervated glands. 5-Hydroxytryptamine was without effect on potassium efflux from either denervated or control glands. The sympathectomy caused increased levels of 5-hydroxytryptamine and 5-hydroxyindole acetic acid as compared with contralateral controls. The results suggest that 5-hydroxytryptamine influences the two main secretory processes in rat parotid gland differently. A significant amylase discharge was seen following 5-hydroxytryptamine stimulation, whereas no effect was seen on ⁸⁶Rb+ efflux. Although it is also proposed that there are no 5-hydroxytryptamine-associated nerves in the superior cervical ganglion innervating parotid tissue, it seems that there is a complex connection between the sympathetic pathway and the serotoninergic system.     

Comparative studies on the effects of aminoglutethimide on hamster and rat adrenal cortex.

Adult female, intact or steroid suppressed hamsters were treated twice daily with 7 mg aminoglutethimide phosphate (AG) for 5 days while intact female rats received 14 mg AG per injection. AG resulted in an increase in adrenal gland weight of both hamster and rat. In the hamster AG had no effect on the amount of lipid droplets while in the rat a slightly higher number of fine lipid vacuoles was seen. In the hamster, enlargement of the gland was due to hyperplasia of the zona reticularis cells while in the rat the number of glomerulosa and fasciculata cells increased. AG had no effect on the adrenal cortex of steroid suppressed hamsters. The serum cortisol level was markedly higher in AG-treated hamsters while the corticosterone level was notably lower in AG-administered rats. In both hamsters and rats, AG-treatment did not change the serum ACTH level. Thus the study demonstrated different responses of the hamster and rat adrenal cortex to AG.     

Some electrical properties of the rabbit anococcygeus muscle and a comparison of the effects of inhibitory nerve stimulation in the rat and rabbit

1. Simultaneous recordings of mechanical activity and membrane potential of individual smooth muscle cells have been made in the rabbit anococcygeus muscle and the effect of field stimulation on these examined.2. In the absence of tone the mean resting membrane potential was - 48 mV. In the stretched muscle spontaneous tone and rhythmic activity quite frequently appeared and this was associated with depolarization of the muscle cells.3. The response to field stimulation depended on the frequency of stimulation, the level of membrane potential and the presence of myogenic tone. The usual response to single pulses or low frequency stimulation was a hyperpolarization of up to 30 mV (mean 14+/-6.8 mV) after a latency of 185 msec and accompanied by muscle relaxation. Higher frequencies (over 8 Hz) produced an initial depolarization often with a spike potential and followed by hyperpolarization. The mechanical response in these instances was contraction or contraction followed by relaxation. At all frequencies rebound depolarization and an associated contraction followed the end of stimulation).4. Phentolamine (5x10(-6)M) and guanethidine (10(-6)M) blocked the initial depolarization and contraction but had no effect on hyperpolarization, muscle relaxation or rebound depolarization and contraction.5. The effect of field stimulation in the presence of guanethidine (4x10(-5)M) was re-examined in the rat anococcygeus. Single pulses were ineffective, repetitive stimulation produced muscle relaxation but no hyperpolarization comparable to the rabbit. Any oscillations in membrane potential were damped during field stimulation and sometimes a small hyperpolarization was produced with a maximum amplitude of 13 mV and a mean of 1.9+/-1.2 mV.6. The transmembrane potential at the peak of hyperpolarization in the rabbit was rarely more than -70 mV. Passive displacement of the membrane potential by current pulses altered the amplitude of the hyperpolarization and suggested that there was a reversal potential at between -80 and -90 mV.7. No change in input resistance could be measured during inhibitory nerve stimulation in either the rabbit or the rat but measurements based on electrotonic potentials indicated a reducation in membrane resistance, small in the rat but greater in the rabbit.8. These experiments suggest that in both species muscle relaxation is associated with an increase in ionic permeability and a move, at least in the rabbit muscle, towards an equilibrium potential of -80 to -90 mV. In view of the much smaller effect in the rat it is not clear whether this is the cause or at least the sole cause of the muscle relaxation.