Smooth Muscle Of Guinea-pig Stomach Research Articles

On the ability of domperidone to selectively inhibit catecholamine-induced relaxation of circular smooth muscle of guinea-pig stomach.

The effects of noradrenaline and dopamine, and their interactions with alpha- and beta-adrenoceptor antagonists and with domperidone, were studied on circular smooth muscle strips taken from the cardia, fundus, body and antrum of the guinea-pig stomach. Noradrenaline and dopamine caused relaxations of all tissues which were generally susceptible to antagonism by either propranolol or phentolamine in concentrations shown to antagonize the relaxations caused by isoprenaline or phenylephrine respectively. In addition, dopamine, in concentrations subthreshold for relaxation, caused contraction of the muscle strips which increased in intensity from the cardia to the antral region: these contractions were antagonized by phentolamine and yohimbine but were insensitive to prazosin: prazosin selectively inhibited the phenylephrine relaxations. With the exception of a modest reduction in the responses of the cardia to dopamine, all tissue responses to noradrenaline and dopamine were resistant to reserpine. Domperidone and haloperidol were found to selectively inhibit the phenylephrine- noradrenaline- and dopamine-induced relaxations of the stomach strips and to enhance the contractile component of dopamine's action: this ability of domperidone to facilitate a dopamine induced contraction, which was most marked in the body and antral regions, was prevented by phentolamine. It is thus concluded that domperidone antagonizes noradrenaline- and dopamine-induced relaxations at one adrenoceptor site having characteristics consistent with an alpha 1-adrenoceptor type whilst failing to antagonize at a further dopamine-sensitive adrenoceptor site involved in contraction of circular smooth muscle of the stomach and having characteristics consistent with an alpha 2-adrenoceptor.

Catecholamines act as alpha 2-adrenoceptors to cause contraction of circular smooth muscle of guinea-pig stomach.

Circular smooth muscle strips taken from the body region of the guinea-pig stomach responded to dopamine and noradrenaline with contraction at lower concentrations followed by relaxation at higher concentrations. A beta-adrenoceptor-mediated relaxation response was excluded by propranolol treatment and this allowed the remaining alpha-adrenoceptor involvement with relaxation and contraction to be incisively differentiated in terms of two distinct alpha-adrenoceptor mechanisms. Thus, the relaxation responses to the catecholamines were mimicked by phenylephrine and antagonized by prazosin, phentolamine but not by yohimbine or rauwolscine. In contrast, the catecholamine-induced contractions were mimicked by clonidine and antagonized by yohimbine and phentolamine but not by prazosin. It is therefore concluded that the alpha mechanisms via which dopamine and noradrenaline are able to relax and contract the circular smooth muscle from the body region of guinea-pig stomach are of the alpha 1- and alpha 2-type respectively.

Characterization of the effect of penehyclidine hydrochloride on muscarinic receptor subtypes mediating the contraction of guinea-pig isolated gastrointestinal smooth muscle

AbstractObjectivesThe aim was to characterize the effect of penehyclidine hydrochloride, which mediates the relaxation of guinea-pig isolated gastrointestinal smooth muscle, on muscarinic receptor subtypes.MethodsRadioimmune assay was used to determine cAMP levels in isolated guinea-pig gastrointestinal smooth muscle to compare the selective effects of penehyclidine hydrochloride on muscarinic receptor subtypes.Key findingsThe results indicated that the relaxing effect of penehyclidine hydro-chloride on isolated gastrointestinal smooth muscle contraction induced by acetylcholine was stronger than that of atropine (based on PA2 values). In the radioimmune assay, penehyclidine hydrochloride increased the cAMP content in isolated guinea-pig stomach smooth muscle and decreased the cAMP content in isolated guinea-pig intestinal smooth muscle, but the difference was not statistically significant at a dose of 10 μmol/1.ConclusionsThe results suggest that penehyclidine hydrochloride has little or no effect on M2 receptor subtypes in guinea-pig gastrointestinal smooth muscle.

Characterization of the effect of penehyclidine hydrochloride on muscarinic receptor subtypes mediating the contraction of guinea-pig isolated gastrointestinal smooth muscle

The aim was to characterize the effect of penehyclidine hydrochloride, which mediates the relaxation of guinea-pig isolated gastrointestinal smooth muscle, on muscarinic receptor subtypes. Radioimmune assay was used to determine cAMP levels in isolated guinea-pig gastrointestinal smooth muscle to compare the selective effects of penehyclidine hydrochloride on muscarinic receptor subtypes. The results indicated that the relaxing effect of penehyclidine hydrochloride on isolated gastrointestinal smooth muscle contraction induced by acetylcholine was stronger than that of atropine (based on PA2 values). In the radioimmune assay, penehyclidine hydrochloride increased the cAMP content in isolated guinea-pig stomach smooth muscle and decreased the cAMP content in isolated guinea-pig intestinal smooth muscle, but the difference was not statistically significant at a dose of 10 mumol/l. The results suggest that penehyclidine hydrochloride has little or no effect on M2 receptor subtypes in guinea-pig gastrointestinal smooth muscle.

Muscarinic activity modulated by C-type natriuretic peptide in gastric smooth muscles of guinea-pig stomach

Natriuretic peptides (NPs) are a cyclic guanosine monophosphate (cGMP) generation system like nitric oxide (NO) and play an inhibitory regulation in gastrointestinal motility but the effect of NPs on muscarinic activity is still unclear. This study was designed to investigate effect of C-type natriuretic peptide (CNP) on muscarinic control of gastric motility and its ion channel mechanism. The spontaneous contraction of gastric smooth muscle strip was recorded by using physiograph in guinea-pig. Membrane currents and potential were recorded by using whole-cell patch-clamp technique. CNP significantly inhibited muscarinic M receptor agonist carbachol (Cch)-induced contractions of gastric smooth muscle strips and dramatically hyperpolarized Cch-induced depolarization of membrane potential in gastric single smooth muscle cell. Muscarinic currents induced by both Cch and GTPγS, a G-protein agonist were significantly suppressed by CNP. 8-Br-cGMP mimicked the effect of CNP on Cch-induced muscarinic currents, and the peak holding current was decreased from − 200.66 ± 54.35 pA of control to − 67.35 ± 24.82 pA. LY83583, a guanylate cyclase nonspecific inhibitor, significantly weakened the inhibitory effect of CNP on muscarinic current while zaprinast, a cGMP sensitive phosphoesterase inhibitor, potentiated the inhibitory effect of CNP on muscarinic current. cGMP production was dramatically enhanced by CNP and this effect was suppressed by LY83583 in gastric smooth muscle. These results suggest that CNP modulates muscarinic activity via CNP–NPR–particulate guanylate cyclase (pGC)–cGMP pathway in guinea-pig.

ATP-sensitive K(+) channels composed of Kir6.1 and SUR2B subunits in guinea pig gastric myocytes.

This study was designed to identify the single-channel properties and molecular entity of ATP-sensitive K(+) (K(ATP)) channels in guinea pig gastric myocytes with patch-clamp recording and RT-PCR. Pinacidil and diazoxide activated K(ATP) currents in a glibenclamide-sensitive manner. The open probability of channels was enhanced by the application of 10 microM pinacidil from 0.085 +/- 0.04 to 0.20 +/- 0.05 (n = 7) and was completely blocked by 10 microM glibenclamide. Single-channel conductance was 37.3 +/- 2.5 pS (n = 4) between -80 and -20 mV in symmetrical K(+) gradient conditions. In inside-out mode, K(ATP) channels showed no spontaneous openings and were activated by the application of nucleotide diphosphates to the cytoplasmic side. These single-channel properties are similar to those of the nucleotide diphosphate-dependent K(+) channels in vascular smooth muscle, which are composed of Kir6.1 and sulfonylurea receptor (SUR)2B. RT-PCR demonstrated the presence of Kir6.1, Kir6.2, and SUR2B in guinea pig stomach smooth muscle cells. These results suggest that K(ATP) channels in smooth muscle cells of the guinea pig stomach are composed of Kir6.1 and SUR2B.

Intact smooth muscle metabolism: its responses to cyanide poisoning and pyruvate stimulation.

Smooth muscle mitochondria are unique in their compartmentalization of metabolism with the contractile proteins and putative role in cell fate choice. In this study we examine the relative and quantitative differences that smooth muscle mitochondria have with regard to cyanide inhibition. The effect of cyanide poisoning in the mitochondria of smooth muscle was examined in the intact porcine carotid artery and intact guinea pig stomach. The respiratory responses of these tissues were monitored in the presence of cyanide and following the addition of various metabolites. In HEPES buffer with 10 mM glucose as the substrate, it was found that the EC50 for cyanide inhibition was 0.11+/-0.02 and 0.14+/-0.02 mM in the pig carotid and guinea stomach respectively. We also found that the signaling metabolite, pyruvate could partially reverse this inhibition. With pyruvate (10 mM) as the substrate, the EC50 increased significantly to 6.52+/-0.11 (carotid artery) and 1.95+/-0. 30 (stomach) mM as well as being significantly different between the tissues. This apparent resistance to cyanide inhibition caused by pyruvate was lost when sodium bicarbonate buffer was used (EC50 in the presence of pyruvate of 0.13+/-0.04 and 0.25+/-0.04 for carotid and stomach respectively). HEPES buffer permitted pyruvate's protection from cyanide poisoning whereas bicarbonate buffer did not. The rate of respiration caused by pyruvate re-stimulation was not significantly different than control in the stomach (2.63+/-0.77 and 2.69+/-0.3??Mol O2/min/g dw respectively) but was significantly greater in the carotid artery. Therefore, smooth muscle with 10 mM pyruvate and 1 mM cyanide had a rate or respiration significantly greater than with 10 mM glucose alone (0.90+/-0.2 and 0.59+/-0. 06??Mol O2/min/g dw respectively). Using 31P NMR spectroscopy, we observed a complete normalisation of high energy phosphates and pH in the guinea pig stomach smooth muscle caused by pyruvate after cyanide poisoning. These results suggest that cyanide's toxicity of smooth muscle oxidative metabolism is affected by the buffer (HEPES versus Bicarbonate) and metabolic signalling molecules or substrates (pyruvate versus glucose) in which the tissues are exposed to as well as tissue to tissue variations.

Consequences of metabolic inhibition in smooth muscle isolated from guinea-pig stomach.

1. In smooth muscle isolated from the guinea-pig stomach, cyanide (CN) and iodoacetic acid (IAA) were applied to block oxidative phosphorylation and glycolysis, respectively. Effects of IAA on generation of spontaneous mechanical and electrical activities were systematically investigated by comparing those of CN. Spontaneous activity ceased in 10-20 min during applications of 1 mM IAA. On the other hand, application of 1 mM CN also reduced the spontaneous activity, but never terminated it. In the presence of CN the negativity of the resting membrane potential was slightly reduced. 2. When spontaneous activity ceased with IAA, the resting membrane potential was not significantly affected. Also, before ceasing, the amplitude and duration of the spontaneous electrical activity were significantly reduced. The amplitude of the electrotonic potential was, however, not changed by IAA. Further, glibenclamide did not prevent the effects of IAA. These results suggest that, unlike cardiac muscle, activation of metabolism-dependent K+ channels in stomach smooth muscle does not seem to play a major role in reducing and terminating spontaneous activity during metabolic inhibition. 3. Carbachol-induced contraction transiently increased, and subsequently decreased gradually during application of IAA. 4. After 50 min application of IAA, when there was no spontaneous activity, the concentrations of phosphocreatine (PCr) and ATP measured with 31P nuclear magnetic resonance decreased to 60 and 80% of the control, respectively, while inorganic phosphate (Pi) concentration paradoxically fell to below detectable levels. During subsequent prolonged application of IAA, high-energy phosphates steadily decreased. On the other hand, after 50 min CN application, [PCr] and [ATP] decreased to approximately 30 and 80% of the control, respectively, while [Pi] increased by 2.6-fold. 5. In the presence of either CN or IAA, spontaneous mechanical and electrical activities were reduced or eliminated, although amounts of high-energy phosphates sufficient to contract smooth muscle remained. It can be postulated that some mechanism(s) related to energy metabolism, but not including ATP-sensitive K+ channels, plays an important role in generating spontaneous activity in guinea-pig stomach smooth muscle. During metabolic inhibition the energy metabolism-dependent mechanism(s) would preserve high-energy phosphates, and consequently cell viability, by stopping spontaneous activity.

Characterization of CCK receptors in stomach smooth muscle: evidence for two subtypes

Cholecystokinin (CCK) and related peptides such as gastrin are important regulators of gastric smooth muscle contraction. Several studies have shown that these effects of CCK and gastrin are mediated by CCK B receptors. However, recent studies suggest the expression of an additional CCK receptor subtype distinct from CCK B receptors in this tissue. This study was designed to distinguish between CCK A and CCK B receptors on guinea-pig stomach smooth muscle cells and to evaluate these cells for additional receptor subtypes. We cloned these receptors by hybridization screening of a guinea-pig smooth muscle cDNA library using 32P random primed labeled cDNA probes from the recently cloned rat CCK A and CCK B receptor coding regions. In addition to clones representing the CCK B subtype, clones of CCK A receptor subtype, but no additional CCK receptor subtypes, could be identified. All isolated clones displayed highly homologous nucleotide sequences in comparison to previously characterized CCK A and CCK B receptors from different species. The results of cDNA hybridization at different levels of stringency and Southern blot analysis using guinea-pig genomic DNA suggest that it is unlikely that additional CCK receptors despite CCK A and CCK B receptors exist in stomach smooth muscle.

Excitation and inhibition by cisapride of spontaneous and evoked electrical responses in smooth muscles of guinea-pig stomach

In isolated circular smooth muscles of the guinea-pig stomach, cisapride depolarized the membrane, increased the amplitude and interval of slow waves, and enhanced the cholinergic excitatory junction potential, with no change in the nonadrenergic noncholinergic inhibitory junction potentials. Methysergide mimicked the excitatory actions of cisapride on junction potentials. Results showed that cisapride has dual actions on gastric muscles; excitation by facilitating the release of ACh possibly via 5-HT receptor blockade and also by depolarizing the smooth muscle membrane, and inhibition by reducing the frequency of slow waves, possibly by acting directly on the pacemakers.

Effects of cromakalim on the contraction and the membrane potential of the circular smooth muscle of guinea-pig stomach.

1. The effects of cromakalim on mechanical and electrical activities of the circular smooth muscles of guinea-pig stomach antrum were observed. 2. Cromakalim (greater than 1 x 10(-7) M) decreased the amplitude of spontaneous rhythmic contractions and also the acetylcholine-enhanced spontaneous contractions. Cromakalim was less effective against the 25.9 mM and 35.9 mM K(+)-induced tonic contractions. 3. Glibenclamide (1 x 10(-6) M) itself caused no detectable change in the spontaneous contractions, those potentiated by acetylcholine or tonic contractions induced by high K+ solutions, but attenuated the actions of cromakalim. On the other hand, charybdotoxin (3 x 10(-8) M) increased the amplitude of spontaneous contractions but failed to affect the actions of cromakalim. 4. Cromakalim (greater than 1 x 10(-6) M) decreased the amplitude and duration of slow waves, and hyperpolarized the membrane. These actions of cromakalim were completely antagonized by 1 x 10(-6) M glibenclamide, whereas part of the effects of cromakalim on mechanical activity was resistant to glibenclamide. 5. The results suggest that the inhibition by cromakalim of the electrical activity and the hyperpolarization, which may be associated with the opening of glibenclamide-sensitive K+ channel, are responsible for its inhibitory action on circular smooth muscle of guinea-pig stomach. Further, some effects independent of glibenclamide-sensitive K+ channel may also be responsible for the mechanical effect.

The sources of calcium for carbachol-induced contraction in the circular smooth muscle of guinea-pig stomach

1. The action of carbachol on the mechanical activity of circular muscle from guinea-pig upper stomach was studied. High concentrations of carbachol (e.g. 10(-4) M) produced a rapid phasic contraction followed by a smaller, sustained tonic contraction. Low concentrations (e.g. 10(-7) M) caused a contraction which did not generally show marked distinction between phasic and tonic components. 2. The response to 10(-7) M carbachol was very sensitive to 10(-5) M nifedipine as was the phasic response to 10(-4) M carbachol. The tonic contraction to the latter, however, was only slightly reduced by nifedipine. 3. The carbachol-induced contractions remaining in the presence of nifedipine were dose-related and very dependent on the presence of external calcium. 4. Carbachol, 10(-7) M, did not produce a contraction after 4 min exposure to calcium-free solution whereas 10(-4) M carbachol did and this was phasic in nature but much reduced relative to the control in normal Ca. 5. A phasic followed by a small tonic contraction to 10(-4) M carbachol was seen superimposed on the K contracture in tissues depolarized with 100 mM K, whereas only a small tonic response occurred for 10(-7) M carbachol. 6. In the absence of a functional carbachol-sensitive intracellular store, 10(-4) M carbachol was unable to trigger a contraction in calcium-free solution. However, when calcium was simultaneously readmitted with carbachol after exposure to calcium-free solution, a contraction occurred. 7. Carbachol, 10(-7) M, did not significantly increase inositol polyphosphate levels, whereas 10(-4) M carbachol did. The increase with 10-4M occurred rapidly peaking within 2min and was undetectable after 5 min, in the absence of lithium. 8. It is concluded that low concentrations of muscarinic agonist trigger a contraction predominantly through a nifedipine-sensitive route whereas higher concentrations further utilize intracellular calcium release and a receptor-operated extracellular calcium-dependent pathway. The former is probably associated with the phasic component and the latter with the tonic one.

Effects of ouabain and EGTA on the inhibitory effect of Ca 2+ on guinea pig stomach smooth muscle (‘Ca reversal’)

Guinea pig stomach smooth muscle is known to show sustained contraction when treated with carbachol (CCh) in Ca-free solution containing 2 × 10 −3 M EGTA (‘Ca-free contraction’). After pretreatment of the muscle with a Ca antagonist (D-600, nicardipine or nifedipine), addition of 10 −4 M Ca 2+ (pCa 4.0) relaxed this CCh-induced Ca-free contraction (‘Ca reversal’), but without pretreatment with a Ca antagonist, Ca 2+ evoked contraction. In a Ca buffer containing 2 × 10 −3 M EGTA, low concentrations of Ca 2+, 10 −8 − 3 × 10 −7 M (pCa 8.0–6.5), relaxed the Ca-free contraction dose dependently (‘Ca reversal’), whereas concentrations higher than 10 −6 M Ca 2+ (pCa 6.0 −) reversed this relaxation back to contraction. EGTA at 5 × 10 −3 or 10 −2 M potentiated the Ca reversal, whereas no reversal occurred in a Ca buffer containing 10 −3 M EGTA. This Ca reversal was inhibited in K +-depleted, Ca-free solution and by ouabain in Ca buffer containing EGTA (5 × 10 −3 or 10 −2 M). These results show that Ca reversal of guinea pig stomach smooth muscle occurs at low concentrations of Ca 2+ and that its mechanism is related to ouabain-sensitive processes, such as the function of Na +-K + ATPase.

P-39 - The effects of ouabain and EGTA on Ca reversal of guinea pig stomach smooth muscle

P-39 - The effects of ouabain and EGTA on Ca reversal of guinea pig stomach smooth muscle

Demarcation of Ca2+ transport processes in guinea pig stomach smooth muscle.

Microsomal fractions were isolated from gastric antrum and fundus smooth muscle of guinea pigs. Ca2+ uptake into and Ca2+ release from the membrane vesicles were studied by a rapid filtration method, and Ca2+ transport properties of the different regions of the stomach were compared. ATP-dependent Ca2+ uptake was similar in microsomes isolated from both regions. This uptake was increased by oxalate and was not affected by NaN3. Oxalate affected Ca2+ permeability of both antrum and fundus microsome vesicles similarly. Fundus microsome vesicles preincubated in 100 mM NaCl and then diluted to 1/20 concentration with Na+-free medium had significantly higher ATP-independent Ca2+ uptake than vesicles preincubated in 100 mM KCl and treated the same way. This was not true for antrum vesicles. Monensin abolished Na+-dependent Ca2+ uptake, and NaCl enhanced Ca2+ efflux from fundus microsome vesicles. The halflife values of Ca2+ loss from fundus vesicles in the presence of NaCl were significantly smaller than those in the presence of KCl. The release of Ca2+ from the vesicles within the first 3 min was accelerated by NaCl to three times that by KCl. However, NaCl had no effect on Ca2+ release from antrum microsome vesicles. Results suggest two distinct mechanisms of stomach membrane Ca2+ transport: (1) ATP-dependent Ca2+ uptake and (2) Na+-Ca2+ exchange; the latter in the fundus only.

Characterization of Ca 2+ transport and enzyme activity in microsomes isolated from guinea-pig stomach smooth muscle

1. 1. Ca 2+ uptake by microsomes prepared from guinea-pig stomach required the presence of both ATP and Mg 2+ and was unaffected by NaN 3. 2. 2. ATP-dependent Ca 2+ uptake increased with increasing free Ca 2+ concentration from 0.1 to 5μM and further increase in Ca 2+ concentration above 5μM did not enhance the uptake further. Half-saturation occurred at approximately 0.55μM. 3. 3. The t 1 2 values of Ca 2+ loss from these vesicles loaded in the presence of oxalate were significantly slower than those in the absence of oxalate. 4. 4. Enzyme activity suggested linkage between Ca 2+ uptake and ATPase activity and most of the azide-sensitive component of ATP hydrolysis was attributable to potent inhibition of ADPase activity.

Neuronally mediated contraction responses of guinea-pig stomach smooth muscle preparations: Modification by benzamide derivatives does not reflect a dopamine antagonist action

The actions of the substituted benzamide derivatives metoclopramide, clebopride, YM-09151-2, tiapride, (+)- and (−)-sulpiride and (+)- and (−)-sultopride, and the dopamine antagonists haloperidol and domperidone, were studied on the responses to field stimulation (0.125–10 Hz) of smooth muscle strips taken from cardia, fundus, body and antral regions of the longitudinal and circular muscle of guinea-pig stomach. Field stimulation of the longitudinal strips caused contraction responses which were antagonised by atropine (but not by prazosin, yohimbine, propranolol or methysergide) to indicate a muscarinic cholinergic involvement. Antagonism of the contractions revealed or enhanced relaxation responses mediated via unidentified mechanisms (resistant to cholinergic and adrenergic antagonists). Metoclopramide enhanced the field stimulation-induced contractions of the stomach smooth muscle preparations via atropine sensitive mechanisms but failed to attenuate the field stimulation-induced relaxation responses. Clebopride's action closely followed that of metoclopramide but YM-09151-2 only enhanced the contraction responses of the longitudinal muscle preparations. Other dopamine antagonists, (+)- and (−)-sulpiride, (+)- and (−)-sultopride, tiapride, haloperidol and domperidone failed to facilitate contraction to field stimulation of any stomach tissue. Thus, the actions of metoclopramide, clebopride and YM-09151-2 to facilitate contraction to field stimulation of stomach smooth muscle are mediated via muscarinic cholinergic mechanism and are not the consequence of an antagonism at any recognisable dopamine receptor.

P-115) - ATPase Activity and Uptake and Release of Ca2+ of Microsome Isolated from Guinea-Pig Stomach Smooth Muscle

P-115) - ATPase Activity and Uptake and Release of Ca2+ of Microsome Isolated from Guinea-Pig Stomach Smooth Muscle

Field stimulation-induced responses of circular smooth muscle from guinea-pig stomach. Characterisation of the receptor mechanisms involved.

Field stimulation of circular smooth muscle of guinea-pig stomach from the regions of the cardia and fundus caused contraction responses at low stimulation frequencies (0.25–1 Hz) with relaxation at higher frequencies (1–10 Hz), whilst tissues of the body and antrum responded with contraction throughout the frequency range. Atropine (10−9–10−8 M) antagonised the contraction responses of all tissues, with relaxation developing at higher concentrations (except for antral tissue). In contrast, metoclopramide (10−8–10−6 M) caused modest (cardia, fundus) or marked (body, antrum) enhancement of contractions to field stimulation, whilst domperidone (10−8–10−7 M), haloperidol (10−8–10−6 M), prazosin, propranolol and methysergide (10−8–10−6 M) failed to modify the contraction responses. However, whilst yohimbine and guanethidine failed to modify the contractions of the cardia, fundus and body tissues, those of the antral preparations were antagonised by nanomolar concentrations of yohimbine and by guanethidine (10−6–5×10−5 M). To optimise the relaxation responses for study, atropine was included in the physiological solution. Relaxation to field stimulation of preparations from the body and cardia, but not the fundus, was antagonised by reserpine pretreatment (5 mg/kg i.p., 24h), addition of guanethidine (10−5–10−4 M), phentolamine, prazosin or propranolol (10−7–10−6 M) (the effects of prazosin and propranolol being additive). Higher concentrations of haloperidol and domperidone antagonised the relaxation responses of the body preparations only. Metoclopramide, yohimbine and methysergide (10−8–10−6 M) were ineffective. Thus, it is concluded that the contractile effects of the 4 stomach areas to field stimulation reflects a major cholinergic involvement, with an additional α2-adrenoceptor contractile component in antral tissue. Relaxation responses of cardia and body tissue involve α2- and β-adrenoceptors plus a further, unidentified, non-adrenergic component; the latter represents the total relaxation response of the fundic preparation.

ATPase activity and calcium uptake of microsomes isolated from stomach smooth muscle after exposure to phospholipase C.

ATPase activity and Ca2+ uptake were examined in microsomal membrane fractions isolated from guinea pig stomach smooth muscle which had been exposed to phospholipase C (PLC). Basal Mg2+-ATPase, Na+, K+-ATPase and Ca2+, Mg2+-ATP activities were inhibited in a time dependent manner by PLC treatment. There was positive correlations between each of these ATPase activities and total phospholipid content of the microsomal fraction. Phosphotidylcholine restored Ca2+, Mg2+-ATPase activity of the microsomal fraction isolated from the tissue which had been treated with PLC for 30 min but not after 60 min. Ca2+ uptake in the presence of ATP by microsomal fraction from tissue treated with PLC for 60 min was significantly decreased. The results provide a cellular basis for the inhibitory effect of PLC on contractility of stomach smooth muscle.