Responses Of Guinea-pig Atria Research Articles

Blockade by 4-Aminopyridine of the Muscarinic-receptor-mediated Responses of Guinea-pig Atria and Trachea

The potassium channel blocker 4-aminopyridine (4-AP) has been shown to antagonize the negative inotropic effects of muscarinic receptor agonists on atrial preparations. This is consistent with muscarinic agonists mediating their negative inotropy through the opening of potassium channels. In the present study, the possibility of a direct antagonism of the muscarinic receptor was examined by comparing the effects of 4-AP upon the responses to carbachol of isolated left atria (negative inotropy) and tracheal spirals (contraction) from reserpine pretreated guinea-pigs. The latter response is K+ channel-independent. The concentration-response curve for carbachol on the paced left atria was displaced 520-fold to the right by 4-AP (10 mM). 4-AP alone caused dose-related contractions of the tracheal spirals. Carbachol-induced contractions were, however, superimposed upon the raised tone and there were substantial rightwards shifts of the concentration-response curves of 4.7- and 31.4-fold by 1 and 10 mM of 4-AP, respectively. Thus 4-AP appears to have muscarinic receptor antagonistic blocking properties. The blockade of the atrial responses was, however, substantially greater and could be attributed to an additional blockade of muscarinic receptor-linked potassium channels. The negative inotropic responses of the A1-adenosine receptor agonist L-phenylisopropyladenosine (L-PIA) were also antagonized by 4-AP, but to a lesser extent than for carbachol. After allowing for the muscarinic receptor blocking activity of 4-AP, carbachol was still antagonized more effectively than L-PIA. This could be due to the presence of a K+ channel-independent component in the response to L-PIA which is not susceptible to 4-AP.

Central endogenous histamine modulates sympathetic outflow through H3 receptors in the conscious rabbit.

1. This study examined the role of histamine H(3) receptors in vagal and sympathetic autonomic reflexes in the conscious rabbit, and in rabbit and guinea-pig isolated right atria. 2. The baroreceptor-heart rate reflex (baroreflex), Bezold-Jarisch-like and nasopharyngeal reflexes were assessed after these treatments (i.v.; with H(1) and H(2) receptor block): (i) vehicle (saline; n=11); (ii) H(3) receptor agonist, (R)-alpha-methylhistamine (R-alpha-MH) 100 micro g kg(-1)+100 micro g kg(-1) h(-1) (n=9); (iii) H(3) receptor antagonist, thioperamide 1 mg kg(-1)+1 mg kg(-1) h(-1) (n=11); (iv) R-alpha-MH and thioperamide (n=6); and (v) H(2) and H(3) antagonist, burimamide 6.3 mg kg(-1)+6.3 mg kg(-1) h(-1) (n=4). 3. R-alpha-MH caused a thioperamide-sensitive fall in mean arterial pressure (MAP) of 8+/-1 mmHg and tachycardia of 18+/-2 bpm (P<0.0005). Burimamide was without effect, however thioperamide elicited an increase in MAP of 4+/-1 mmHg (P<0.01), but no change in heart rate (HR). 4. R-alpha-MH caused a 44% decrease in the average gain of the baroreflex (P=0.0001); this effect was antagonised by thioperamide. Thioperamide caused a parallel rightward shift in the barocurve with an increase in MAP of 5 mmHg (P<0.05). Burimamide had no effect on the baroreflex. The vagally mediated bradycardia elicited by the Bezold-Jarisch and nasopharyngeal reflexes was unaffected by H(3) receptor ligand administration. 5. R-alpha-MH (<or=10 micro M) caused a thioperamide-sensitive depression of both sympathetic and vagal responses in guinea-pig atria, but had no effect in rabbit atria. 6. As H(3) receptor activation caused a significant decrease in baroreflex gain without affecting HR range, the former is unlikely to be simply due to peripheral sympatholysis (supported by the lack of effect in isolated atria). Central H(3) receptors may have a tonic role in the baroreflex as thioperamide caused a rightward resetting of the barocurve. In contrast, the peripherally acting H(3) antagonist burimamide was without effect. These findings suggest a role for central histamine H(3) receptors in cardiovascular homeostasis in the rabbit.

Effects of two truncated forms of human calcitonin-gene related peptide: implications for receptor classification

We investigated the possibility that human α-calcitonin-gene related peptide (CGRP)-(8–37) and human βCGRP-(8–37) show some selectivity as antagonists of CGRP 1 and CGRP 2 receptor-mediated responses. Bindings assays showed that human αCGRP, human αCGRP-(8–37) and human βCGRP-(8–37) showed high affinity (in the nanomolar concentration range) for CGRP receptors expressed in SK-N-MC cells and also in rat brain membrain preparations. Both human αCGRP-(8–37) and human βCGRP-(8–37) were potent antagonists of human αCGRP-stimulated cAMP accumulation in SK-N-MC cells. However, both human αCGRP-(8–37) and human βCGRP-(8–37) were weakly effective in antagonizing human αCGRP-stimulated responses in guinea-pig atria and rat vas deferens. In rat vas deferens, but not guinea-pig atria, the effects of human αCGRP and human αCGRP-(8–37) (but not human βCGRP-(8–37)) were potentiated by thirophan. Neither human α- nor human βCGRP-(8–37) showed selectivity for supposedly CGRP 1 and CGRP 2 receptor-mediated responses. Furthermore, differences in the effects of the truncated CGRP analogues may reflect differences in enzyme distribution rather than the existence of CGRP receptor subtypes.

O-150 - Histamine H2-receptor antagonism of T-593, a novel anti-ulcer drug : Studies on positive chronotropic responses in guinea-pig atria.

O-150 - Histamine H2-receptor antagonism of T-593, a novel anti-ulcer drug : Studies on positive chronotropic responses in guinea-pig atria.

The interaction of parafluorohexahydrosiladiphenidol at muscarinic receptors in vitro

1. The antagonistic actions of parafluorohexahydrosiladiphenidol (pFHHSiD) at muscarinic receptors has been studied in cardiac muscle, smooth muscle and cell culture preparations. In this paper, the classification scheme of Doods et al. (1987) is employed. This scheme is based upon differential affinities of muscarinic antagonists. pFHHSiD exhibited high pA2 values at M3 receptors mediating contractions of guinea-pig ileum and oesophageal muscularis mucosae (7.8 and 8.2 respectively) whereas low values were determined at M2 receptors mediating negative inotropic responses in guinea-pig atria (6.0). Intermediate pA2 values were determined at M1 receptors mediating contractions of the canine femoral and saphenous veins. 2. The pA2 values of pFHHSiD at receptors mediating endothelial-dependent relaxation of rat aortic rings, rabbit jugular vein and canine femoral artery (7.6-7.9) were similar to those determined on the ileum. However, the pA2 values of pFHHSiD at receptors mediating contractions of the guinea-pig trachea (7.1), which has been previously shown to possess M3 receptors, were different from those determined in the ileum. 3. The similarity in pA2 values of pFHHSiD between the M3 receptors in guinea-pig ileum and the receptors mediating endothelial-dependent relaxations provide further evidence for the role of M3 receptors in this vascular response. Taken together, pA2 values for pFHHSiD range from 7.1 to 8.2, depending upon the M3 preparation used. The selectivity of the compound therefore for the M3 versus the M2 muscarinic receptor ranged from 13 to 163 fold. 4. At muscarinic receptors mediating stimulation of phosphatidylinositol hydrolysis, pFHHSiD paradoxically displayed a high affinity for the M1 receptor in the SH-SY5Y cell line (pA2 = 7.9) as well as for the M3 receptor in the human astrocytoma (1321 NI cell line (pA2 = 7.6). The value at the M1 receptor in SH-SY5Y cells was greater than was observed at M1 receptors mediating contractions of both the canine saphenous and femoral veins (7.1). 5. pFHHSiD, therefore, clearly delineated M3 from M2 muscarinic receptors, whilst the separation between M1 and M3 receptors was variable. The reason for the anomalous affinity estimates in some functional studies remains unclear. These data indicate that the pA2 values for pFHHSiD appear to be tissue-dependent since the M3 selectivity varies according to the preparations studied. As a result the utility of pFHHSiD in muscarinic receptor classification is limited.

Desensitization of β-Adrenoceptor-Mediated Functional Responses of Guinea Pig Atria by In Vitro Incubation with Isoprenaline

Positive inotropic and chronotropic responses to (-)-isoprenaline were recorded in guinea pig isolated paced left and spontaneously beating right atria. Concentration-response curves were determined before and after incubation with 10(-6) M (-)-isoprenaline for various times from 0.5 to 8 h. Before construction of the postincubation curve, isoprenaline was washed from the tissue for either 0.5 or 1 h. Preincubation curves were corrected from controls for time-dependent changes in sensitivity. After a 2-h incubation with 10(-6) M isoprenaline and a 0.5-h washout, there was a significant rightward shift of the right atrial rate curve [concentration ratio (CR) 10.6 +/- 2.1]. This was reversed by extending the washout to 1 h. After a 4-h incubation, however, a poorly reversible desensitization occurred, the curve still being significantly shifted after a 1-h washout (CR, 17.4 +/- 6.2). A higher incubation concentration, 10(-5) M, appeared to produce greater desensitization as a depression of the maximum response, however, this was an artifact caused by the failure to wash the isoprenaline from the tissue. When left atria were paced throughout the incubation period, a substantial nonspecific depression of the maximum response to 73.7 +/- 7.6% occurred. When pacing was stopped during incubation, only a rightward shift of the curve was produced by incubation with isoprenaline (10(-6) M), with no change of maximum (96.5 +/- 6.3%). There was no cross-desensitization of histamine in either the left (unpaced while incubating) or right atria after a 4-h incubation period with isoprenaline (10(-6) M). The desensitization was therefore beta-adrenoceptor specific and of the homologous type. After an 8-h incubation, there was an additional depression of the postincubation maximum which was greater in the left (68.9 +/- 2.3%) than right atria (85.2 +/- 5.4%). This true reduction of the maxima indicates that desensitization had effectively removed a proportion of the beta adrenoceptors.

Binding and functional properties of antimuscarinics of the hexocyclium/sila-hexocyclium and hexahydro-diphenidol/hexahydro-sila-diphenidol type to muscarinic receptor subtypes.

1. In an attempt to assess the structural requirements for the muscarinic receptor selectivity of hexahydro-diphenidol (hexahydro-difenidol) and hexahydro-sila-diphenidol (hexahydro-sila-difenidol), a series of structurally related C/Si pairs were investigated, along with atropine, pirenzepine and methoctramine, for their binding affinities in NB-OK 1 cells as well as in rat heart and pancreas. 2. The action of these antagonists at muscarinic receptors mediating negative inotropic responses in guinea-pig atria and ileal contractions has also been assessed. 3. Antagonist binding data indicated that NB-OK 1 cells (M1 type) as well as rat heart (cardiac type) and pancreas (glandular/smooth muscle type) possess different muscarinic receptor subtypes. 4. A highly significant correlation was found between the binding affinities of the antagonists to muscarinic receptors in rat heart and pancreas, respectively, and the affinities to muscarinic receptors in guinea-pig atria and ileum. This implies that the muscarinic binding sites in rat heart and the receptors in guinea-pig atria are essentially similar, but different from those in pancreas and ileum. 5. The antimuscarinic potency of hexahydro-diphenidol and hexahydro-sila-diphenidol at the three subtypes was influenced differently by structural modifications (e.g. quaternization). Different selectivity profiles for the antagonists were obtained, which makes these compounds useful tools to investigate further muscarinic receptor heterogeneity. Indeed, the tertiary analogues hexahydro-diphenidol (HHD) and hexahydro-sila-diphenidol (HHSiD) had an M1 = glandular/smooth muscle greater than cardiac selectivity profile, whereas the quaternary analogues HHD methiodide and HHSiD methiodide were M1 preferring (M1 greater than glandular/smooth muscle, cardiac).

The effects of ICS 205-930, a new 5-HT 3 receptor antagonist, on responses of guinea pig atria

The effects of ICS 205-930, a new 5-HT 3 receptor antagonist, on responses of guinea pig atria

Temperature dependence of β1-adrenoceptor-mediated responses examined by use of partial agonists

The inotropic and chronotropic responses of guinea pig atria, and the relaxation responses of guinea pig intestine, trachea, lung, uterus and vas deferens to catecholamines have been examined at bath temperatures of 38°C and 30°C. Hypothermia resulted in a supersensitivity of cardiac tissues with a decrease in isoprenaline EC 50 and an increase in the maximum response to the partial agonist, salbutamol. Ileum responses to isoprenaline were potentiated at 30°C but no partial agonist could be found on this tissue. Responses of the lung and vas deferens to partial agonists were not affected by temperature, while uterine responses were inhibited by hypothermia. The trachea was supersensitive to isoprenaline at 30°C, however this was not due to a change in β-adrenoceptor sensitivity but an inhibition of COMT. Partial agonist responses of trachea were similar at both temperatures. β-Adrenoceptor supersensitivity was therefore observed only where responses are mediated primarily by β 1-adrenoceptors and supports the concept that β 1- but not β 2-adrenoceptors exhibit hypothermia-induced supersensitivity.

Studies on the H2-receptor antagonism of MK-208 in isolated rabbit gastric glands

Using isolated rabbit gastric glands, the H2-receptor antagonist MK-208 was investigated with respect to its effects on [14C]aminopyrine uptake as an index of gastric acid secretion. In addition to shifting the histamine concentration-response curve to the right in a parallel fashion, the antagonism produced by MK-208 was reversible, contrary to that previously seen in the guinea pig atria. These observations suggest that the H2-receptors responsible for mediating gastric secretion in the rabbit and the chronotropic response in guinea pig atria are different.

??-Blocking Potency of Mepindolol and Pindolol Against Isoproterenol-Induced Inotropic and Chronotropic Responses in Guinea Pig Atria

The beta-blocking potencies of mepindolol and pindolol were determined in guinea pig right atria (heart rate) and paced left atria (contractile force) in vitro. Neither agent showed selectivity for blocking rate responses induced by isoproterenol. The pA2 (95% confidence interval) values determined for mepindolol against rate and force responses induced by isoproterenol were 9.96 (9.91-10.01) and 9.52 (9.44-9.60), respectively, and for pindolol, 9.20 (9.15-9.25) and 8.85 (8.76-8.94). Mepindolol was approximately 6.5 times as potent as pindolol. The present evidence does not support the hypothesis of subpopulations of beta-receptors subserving inotropic and chronotropic responses.

Field-stimulus response of guinea-pig atria as influenced by the peptides angiotensin, bradykinin, and substance P.

Field stimulation produced in isolated guinea-pig left atria a positive inotropic response, which was highly sensitive to the inhibitory actions of lidocaine, tetrodotoxin, and of pindolol. The field-stimulus effect was enhanced by angiotensin and by bradykinin, but not by substance P. Only bradykinin exerted also a direct positive inotropic effect (in the absence of field stimulation). It is concluded that the intramural adrenergic neurons of the guinea-pig atrium have receptors for both angiotensin and bradykinin, but not for substance P.

Time courses of carbachol-induced responses in guinea pig atria under the influence of ouabain, calcium, and rate of stimulation.

The response of isolated left atria of guinea pigs to carbachol was measured in the presence and during washout of the drug. The time courses of the negative inotropic effect and of the rate of shortening in action potential duration were compared. The initial rapid decline in force of contraction was followed by a slowly developing partial recovery (fading). After removal of the drug, a transient phase of hypercontractility was observed before the control level was reached again. In contrast to the mechanical effect, the drug-induced shortening of the action potential duration proceeded monophasically. The fading and hypercontractility phenomena shown by the mechanical response to the drug were influenced by the stimulation rate and by the presence of ouabain. From the dissociation of electrical and mechanical effects it is concluded that fading and hypercontractility are not due to changes of receptor sensitivity but reflect a change in cellular calcium distribution.

Synthesis and biological evaluation of a tetrahydroisoquinoline derivative possessing selective beta2-adrenergic agonist activity.

This paper reports the synthesis of 4-(3,4,5-trimethoxybenzyl)-6,7-dihydroxy-1,2,3,4-tetrahydroisoquinoline (2) and 2-(3,4-dihydroxyphenyl)-3-(3,4,5-trimethoxyphenyl)propylamine (3). The biological activity of these agents relative to that of trimetoquinol (1) in guinea pig atria and guinea pig trachea is reported. The relative activities in relaxation of guinea pig trachea is 1 greater than 2 greater than 3 while in the chronotropic response in guinea pig atria the relative order of activity is 1 greater than 3 greater than 2.

Cardiac inotropism of dimethyl sulphoxide: Osmotic effects and interactions with calcium ion

DMSO is a cryoprotectant drug which produces concentration dependent inotropic and chronotropic changes in a variety of cardiac muscle preparations. Electrically driven and rabbit guinea pig atria were used to elucidate the mechanisms of these changes. DMSO concentrations of 0.14 to 1.41 M generally produced significant positive inotropic responses in guinea pig atria, and significant negative inotropic responses in rabbit atria. The form and magnitude of the response to given concentrations of DMSO depended upon the manner in which DMSO was administered and removed, suggesting that osmotic effects played an important role in producing the responses. Inotropic responses to DMSO appeared to be independent of extracellular Ca 2+ concentration if rabbit atria were equilibrated in the altered Ca 2+ environment prior to addition of DMSO. When atria were placed in Ca 2+-depleted media, the presence of DMSO significantly augmented the rate of loss of contractile tension. Kinetic analysis of contractile tension decay suggested that DMSO acted on a pool of myocardial Ca 2+ responsible for regulating contractile strength. The magnitude of this effect was independent of DMSO concentration, and so may not be a direct result of osmotic mechanisms. These actions on contractile strength will be considered in relation to other reported possible mechanisms of DMSO cardiac actions.