Changes in the sensitivity to cholinomimetic drugs of smooth and cardiac muscle in the rat induced by subacute administration of di-isopropyl phosphorofluoridate

The effect of subacute administration to rats of di-isopropyl phosphorofluoridate (DFP) on the sensitivity of the cardiovascular system to carbachol and on the sensitivity of various isolated tissues to acetylcholine, methacholine and carbachol was investigated. Cholinesterase inhibition produced by DFP in the isolated tissues was also determined.In the cardiovascular experiments both the magnitude of responses and the slope of the dose-response curves for hypotension and bradycardia produced by carbachol in the anaesthetized rat were reduced in the DFP-treated group.In experiments with isolated tissuesfrom DFP-treated animals it was found that sensitivity to methacholine and acetylcholine increased in the bladder to a greater extent that in the atria or ileum. The sensitivity to carbachol decreased to a similar extent in all 3 tissues. Acetylcholinesterase and butyrylcholinesterase levels in the bladder were inhibited to a greater extent than in the other 2 tissues.The results show that the degree of enzyme inhibition produced by DFP in the tissues after subacute administration for 10 days does not correlate with the decrease in the sensitivity of the tissues to carbachol. It is suggested that the association between accumulation of transmitter and enzyme inhibition is a more important factor than the degree of enzyme inhibition itself in the development of subsensitivity to carbachol.The release of cholinergic transmitter from the electrically stimulated isolated rat bladder may also be impaired following subacute administration of DFP.     

Influence of thyroid status on responses of rat isolated pulmonary artery, vas deferens and trachea to smooth muscle relaxant drugs.

1 Responses to relaxant drugs have been examined on isolated KCl-contracted smooth muscle preparations from rats in which thyroid status was changed by prior treatment with either thyroxine (T4) for 1 week (preparations of pulmonary artery, trachea and vas deferens) or methimazole for 10-12 weeks (pulmonary artery preparations). 2 On pulmonary artery preparations, T4 treatment caused a significant increase in the magnitude of the relaxant responses to noradrenaline and isoprenaline but not those to adrenaline. The potency of noradrenaline was increased 5.6 fold but that of isoprenaline and adrenaline was unchanged. This resulted in a change in the relative potencies from adrenaline greater than noradrenaline (controls) to noradrenaline = adrenaline (T4-treated). Methimazole treatment caused a significant reduction in the magnitude of the responses to noradrenaline and in its potency (2.8 fold). Isoprenaline and procaterol were unaffected. 3 On pulmonary artery preparations, T4 treatment did not affect the magnitude of the responses to forskolin, sodium nitrite or isobutylmethylxanthine (IBMX) or their potency. In vitro treatment with the monoamine oxidase (MAO) inhibitors, iproniazid or pargyline, did not potentiate responses to either noradrenaline or isoprenaline. Therefore, it was concluded that the T4-induced changes in the magnitude of the responses to noradrenaline and isoprenaline and in the potency of noradrenaline were unlikely to be due to reduced activity of cyclic nucleotide phosphodiesterase(s) or MAO. 4 On preparations of vas deferens and trachea, T4 treatment had no effect on the magnitude of the responses to noradrenaline, isoprenaline, adrenaline or procaterol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Influence of sympathetic nerves on development of responsiveness of the chick smooth muscle to drugs.

The role of sympathetic nerves in the development of responsiveness of smooth muscle to drugs was investigated using the expansor secondariorum muscle of 2- to 40-day-old chicks. The normal developmental decrease in the responsiveness of the muscle to acetylcholine was prevented by surgical transection of the nerve or chronic treatment with guanethidine, while it was facilitated by chronic treatment with dimethylphenylpiperazinium. Sensitivity of the muscle to noradrenaline, remaining constant during normal development, was increased by nerve section or guanethidine treatment, while it was slightly decreased by dimethylphenylpipe-azinium treatment. These results suggest that sympathetic nerves regulate the development of the responsiveness of the chick expansor secundariorum muscle to drugs, at least the developmental decrease in responsiveness to acetylcholine.     

Psychotherapeutic drugs and biogenic amines. Current concepts and therapeutic implications

Over the last 2 decades evidence has continued to accumulate from studies in various model systems that drugs effective in the treatment of major psychiatric disorders alter biogenic amines which function as neurotransmitters. As a result of these findings, various hypotheses have been formulated that there is a fundamental abnormality of one or another of the biogenic amine systems (i.e. the serotonergic, noradrenergic and dopaminergic systems) in the affective disorders and schizophrenia. Numerous attempts have been made to assess these biogenic amine hypotheses, primarily through quantitation of the major metabolites of the amines and recently by assessment of receptor sensitivity or density. The combination of basic pharmacological and clinical research has shown that there are biochemical subtypes, especially among those with affective illness, which might show a preferential response to biochemically specific drugs. If verified, such research constitutes a significant therapeutic advance. The status of these investigations is critically assessed in this review. Finally, many new techniques, challenge tests and biochemically distinct drugs are being introduced. Promising approaches that are likely to produce refinements of the biogenic amine hypotheses, as well as suggesting alternative formulations, are reviewed.     

Effects of biogenic amines and psychotropic drugs on endogenous prostaglandin biosynthesis in the rat brain homogenates

Phenylalkylamine and indolalkylamine derivatives, as well as several drugs acting on the central nervous system, were tested for their effects on endogenous prostaglandin, (PG) biosynthesis in the rat brain homogenates. In the particulate suspension obtained by the removal of the soluble fraction from the rat brain homogenates PG-biosynthesis could be stimulated by noradrenaline. dopamine, adrenaline, serotonin, tryptamine and to a slight extent by tyramine. Isoprenaline, DOPA. α-methyl noradrenaline, α-methyl dopamine, α-methyl tryptamine and 5-hydroxy tryptophan were ineffective. PG-biosynthesis stimulated by catecholamines or indolalkylamines responsively could be inhibited by compounds with monoamine oxidase blocking properties. In the total rat brain homogenates another type of PG-biosynthesis could be demonstrated in the absence of catecholamine or indolalkylamine that could not, or but to a slight extent, be inhibited by monoamine oxidase blocking agents. Apomorphine, oxypertine, α-methyl noradrenaline, promethazine, DOPA, reserpine, chlorpromazine, desipramine, yohimbine and tetrabenazine inhibited this type of PG-biosynthesis, though they failed to influence PG-formation stimulated by catecholamine or indolalkylamine. A correlation could be established between the PG-formation inhibitory and lipid peroxidation antagonizing effects of these compounds. Non-steroidal anti-inflammatory agents, such as indomethacin, acetylsalicylic acid and dipyrone, inhibited both types of PG-biosynthesis. The results permit the conclusion that psychotropic drugs exert their effects on endogenous PG biosynthesis in the rat brain homogenates by inhibiting various activation processes.     

Comparison of high affinity binding of calcium channel blocking drugs to vascular smooth muscle and cardiac sarcolemmal membranes

The binding of the 1,4-dihydropyridine calcium channel blocker [3H]nitrendipine to canine cardiac sarcolemmal and bovine aortic membranes was found to be rapid, specific, saturable, and reversible. Dissociation constants (Kd) determined by Scatchard analysis were 0.14 and 0.16 nM and the maximal numbers of binding sites (Bmax) were 0.96 +/- 0.2 and 0.08 +/- 0.01 pmole/mg protein for cardiac and aortic membranes respectively. Values of Kd calculated from kinetic data were approximately 0.10 nM for both membrane preparations. Competition assays with the enantiomers of a nisoldipine derivative indicated that [3H]nitrendipine binds stereoselectively. The order of potency of several nifedipine analogs for inhibition of binding of [3H]nitrendipine to cardiac and aortic membranes paralleled their relative potencies for inhibition of contraction in smooth muscle. It is concluded that the high affinity binding sites for nitrendipine in bovine aortic smooth muscle membranes are similar to those of canine ventricular sarcolemma.     

Differential pharmacologic sensitivity of cyclic nucleotide phosphodiesterase isozymes isolated from cardiac muscle, arterial and airway smooth muscle

Phosphodiesterase isozymes were isolated by diethylaminoethyl ether (DEAE) column chromatography from cardiac muscle (canine, guinea pig), vascular (canine and guinea pig aortic) and airway (canine tracheal) smooth muscle. All peak I phosphodiesterases had a low apparent Km (0.29-0.49 microM) for guanosine 3':5' cyclic monophosphate (cGMP) and all peak III phosphodiesterases had a low apparent Km (0.35-0.58 microM) for adenosine 3':5' cyclic monophosphate (cAMP); trachealis peak III also had a high Km for cAMP (32 microM). The potency and selectivity for inhibition of peak I or peak III phosphodiesterase by theophylline and papaverine, the peak I selective inhibitor M + B 22948, and the peak III selective inhibitors amrinone, milrinone, imazodan, CI-930 and piroximone were approximately equal when isozymes isolated from aortic smooth muscle were compared to isozymes isolated from cardiac muscle of both species. Rolipram was relatively potent as a peak III phosphodiesterase inhibitor in canine cardiac muscle, but was impotent in the other cardiovascular peak IIIs. In tracheal smooth muscle, the cardiovascular selective peak III phosphodiesterase selective inhibitors were substantially less potent while rolipram was more potent as a peak III inhibitor. In summary, these studies show that while cardiac and vascular smooth muscle phosphodiesterase isozymes are pharmacologically similar, there is pharmacological and substrate heterogeneity of peak III phosphodiesterase in aortic vs. trachea smooth muscle within the same species.     

Uptake inhibition of biogenic amines by newer antidepressant drugs: Relevance to the dopamine hypothesis of depression

The dopamine theory of depression was studied by assessing the effect of antidepressant drugs on uptake of dopamine, noradrenaline, and serotonin in synaptosomes from rat brain. Five newer drugs—butriptyline, maprotiline, trimipramine, iprindole, and mianserine—exhibited rather potent inhibition of ³H-dopamine uptake in corpus striatum, as their IC₅₀ values, which were in the order of 10^-6–10^-5 M, were only about 50 times higher than for nomifensine (IC₅₀=10^-7 M). The five drugs were weak, compared to chlorimipramine, on ¹⁴C-serotonin uptake in the whole forebrain, as their IC₅₀ were about 10^-5 M. Butriptyline, trimipramine, and iprindole were very weak uptake inhibitors of ³H-noradrenaline in the occipital cortex. Their IC₅₀ values were about 10^-6 M, which is almost 1000 times higher than for desmethylimipramine. These results are discussed in relation to comprehensive recent literature as further indicating a link between dopamine and depression.     

Alternative splicing modulates diltiazem sensitivity of cardiac and vascular smooth muscle Cav1.2 calcium channels

Background and purpose:

As a calcium channel blocker, diltiazem acts mainly on the voltage-gated calcium channels, Ca_v1.2, for its beneficial effects in cardiovascular diseases such as hypertension, angina and/or supraventricular arrhythmias. However, the effects of diltiazem on different isoforms of Ca_v1.2 channels expressed in heart and vascular smooth muscles remain to be investigated. Here, we characterized the effects of diltiazem on the splice variants of Ca_v1.2 channels, predominant in cardiac and vascular smooth muscles.

Experimental approach:

Cardiac and smooth muscle isoforms of Ca_v1.2 channels were expressed in human embryonic kidney cells and their electrophysiological properties were characterized using whole-cell patch-clamp techniques.

Key results:

Under closed-channel and use-dependent block (0.03 Hz), cardiac splice variant Ca_v1.2CM was less sensitive to diltiazem than two major smooth muscle splice variants, Ca_v1.2SM and Ca_v1.2b. Ca_v1.2CM has a more positive half-inactivation potential than the smooth muscle channels, and diltiazem shifted it less to negative potential. Additionally, the current decay was slower in Ca_v1.2CM channels. When we modified alternatively spliced exons of cardiac Ca_v1.2CM channels into smooth muscle exons, we found that all three loci contribute to the different diltiazem sensitivity between cardiac and smooth muscle splice isoforms.

Conclusions and implications:

Alternative splicing of Ca_v1.2 channels modifies diltiazem sensitivity in the heart and blood vessels. Gating properties altered by diltiazem are different in the three channels.     

The action of acetylcholine and other drugs on the efflux of potassium and rubidium from smooth muscle of the guinea-pig intestine

1. A method is described for measuring continuously the efflux of potassium or rubidium from smooth muscle of the guinea-pig.

2. Muscarinic drugs cause at maximum a 100-fold increase in the efflux rate, due to a direct increase in permeability and only to a minor extent secondary to depolarization. With acetylcholine the dose response curve for producing efflux is displaced to 1,000 times higher concentrations than that for contraction.

3. The shift varies with different agonists. The efflux and contractile responses to agonists are antagonized to an equivalent extent by atropine and several other reversible antagonists but benzhexol has a relatively greater effect on efflux. An estimate of spare receptors was obtained with benzilylcholine mustard and was similar for both responses. Dibenamine and local anaesthetics led to a parallel shift of the contraction dose response curve but a depression without shift in the efflux response.

4. The most satisfactory explanation of these results is that there are two types of the muscarinic receptor in the smooth muscle of the guinea-pig intestine.

     

Effect of preload on rat aortic smooth muscle sensitivity to vasoactive agents

Concentration-response curves to potassium chloride, phenylephrine, serotonin and calcium chloride were obtained from rat aortic strips subjected to preloads of 0.75, 1.5 or 3.0 g. The sensitivity of the aortic smooth muscle to potassium chloride, phenylephrine and serotonin increased with increasing preload; whereas the calcium chloride concentration-response curves of K+-depolarized strips were unaffected by preload. These results demonstrate that the sensitivity of rat aortic smooth muscle to many vasoactive agents is a function of preload and also indicate that an alteration in the influx of external Ca2+ is not sufficient to explain the effect of preload on sensitivity.     

Increased sensitivity to nifedipine of smooth muscle from hypertensive rats

The sensitivity of smooth muscle from aortas of spontaneously hypertensive rats (SHR), renal hypertensive rats: two kidney-one clip and one kidney-one clip (2K-1C, 1K-1C) and DOCA salt hypertensive rats to the relaxant effect of nifedipine (NIF) was studied. A parallel leftward shift of the concentration-relaxation curve was detected in the K-precontracted aortic smooth muscle from hypertensive rats. This increased sensitivity seems to be related to the degree of hypertension and independent of the experimental method used to produce the high blood pressure level. No change in sensitivity was detected either in SHR or in renal hypertensive rats when nitroglycerin was used as a vasodilator.     

The effects of calcium and magnesium on the response of intestinal smooth muscle to drugs

1. The sensitivity of the longitudinal muscle of the guinea-pig ileum to muscarinic drugs producing contraction depends on optimum concentrations of calcium and magnesium. It can also be reduced by changes in sodium concentration and osmolarity.2. The rubidium efflux response to these same drugs is insensitive to any of these changes in the external medium.3. Raised calcium or magnesium concentration has the effect of largely annulling the differences in structure-activity relationships of the two responses as they exist in optimal media.4. The effects are explained in terms of a labile coupling process between a single receptor and the contractile process compared with a stable coupling process of the efflux process.