Characterization of voltage-gated calcium currents in freshly isolated smooth muscle cells from rat tail main artery.

The aim of the present study was to characterize voltage-gated Ca2+ currents in smooth muscle cells freshly isolated from rat tail main artery in the presence of 5 mmol L(-1) external Ca2+. Calcium currents were identified on the basis of their voltage dependencies and sensitivity to nifedipine, Ni2+ and cinnarizine. In the majority of the cells studied, T- and L-type currents were observed, while the remaining cells showed predominantly L-type currents. In the latter group of cells, holding potential change from -50 to either -70 or -90 mV increased the corresponding inward current amplitude while its voltage activation threshold remained unchanged. The steady state inactivation of L-type Ca2+ channels showed half-maximal inactivation at -38 mV. A Ca2+-dependent inactivation was also evident. Nifedipine (3 micromol L(-1)) blocked L-type but not T-type Ca2+ currents. Ni2+ (50 micromol L(-1)) as well as cinnarizine (1 micromol L(-1)) suppressed the nifedipine-resistant, T-type component of the currents. At higher concentrations, both Ni2+ (0.3-1 mmol L(-1)) and cinnarizine (10 micromol L(-1)) blocked the net inward current. Replacement of Ca2+ with 10 mmol L(-)1 Ba2+ significantly increased the amplitude of L-type Ca2+ currents. These results demonstrate that smooth muscle cells freshly isolated from rat tail main artery may be divided into two populations, one expressing both L- and T-type and the other only L-type Ca2+ channels. Furthermore, this report shows that in arterial smooth muscle cells cinnarizine potently inhibited T-type currents at low concentrations (1 micromol L(-1)) but also blocked L-type Ca2+ currents at higher concentrations (10 micromol L(-1)).

Control of the phasic and tonic contractions of guinea pig stomach by a ryanodine-sensitive Ca2+ store.

In some smooth muscle cells, the rise in intracellular Ca2+ as a result of a Ca2+ influx via plasma membrane Ca2+ channels can activate a further increase in intracellular Ca2+ as a result of Ca2+ release from intracellular stores. This study examined the role of the Ca2+-induced Ca2+ release from the ryanodine-sensitive intracellular Ca2+ stores in shaping the smooth muscle contractions of guinea pig stomach. The contractile activity of isolated muscle strips of the fundus, corpus and antrum region of the stomach was recorded under isometric conditions. Ryanodine, an activator of Ca2+-induced Ca2+ release, concentration dependently (10(-7)-3x10(-5) M) increased the tone of fundus and corpus strips. Ryanodine had a dual action on the phasic contractions of the antrum and corpus: increase by the low concentrations (up to 10(-6) M) and inhibition by the high concentrations (10(-6)-3x10(-5) M). Nifedipine (10(-5) M) completely inhibited the ryanodine (10(-6) M)-induced phasic contractions and only partly the ryanodine (3x10(-5) M)-induced tonic contractions. In the presence of 10(-5) M cyclopiazonic acid, a specific inhibitor of sarcoplasmic reticulum Ca2+-ATPase, ryanodine (3x10(-5) M) further increased the tone of the corpus and fundus strips. Ryanodine (3x10(-5) M) induced tonic contractions in the fundus and corpus precontracted by acetylcholine (10(-5) M), and inhibited the acetylcholine (10(-6) M)-induced phasic contractions in the antrum and corpus. Ruthenium red, an inhibitor of Ca2+-induced Ca2+ release, concentration dependently (10(-6)-10(-4) M) decreased the tone and amplitude of the phasic contractions. The data obtained provide evidence for the participation of a sarcoplasmic reticulum Ca2+-induced Ca2+ release mechanism in shaping the tonic and phasic contractions of guinea pig stomach, and highlight important tissue differences.

The role of a ryanodine-sensitive Ca(2+)-store in the regulation of smooth muscle tone of the cat gastric fundus.

The smooth muscle of the gastric fundus maintains spontaneous tone, but the mechanism underlying this activity is not fully understood. The aim of the present study was to examine whether Ca2+ release from the sarcoplasmic reticulum (SR) could play a role in the maintenance of the spontaneous smooth muscle tone of the cat gastric fundus. The effects on the contractile activity of SR Ca2+ release activators ryanodine and caffeine and of the inhibitor ruthenium red were studied. The contractile activity of isolated muscle strips was recorded under isometric conditions using organ baths. Ryanodine concentration-dependently (10(-7)-3.10(-5) mol/l) increased the tone of the fundus strips. In the presence of nifedipine (10(-5) mol/l), ryanodine induced a nifedipine-resistant tonic contraction. The tonic contraction induced after inhibition of the SR Ca(2+)-pump by cyclopiazonic acid was potentiated by ryanodine (10(-5) mol/l). In strips precontracted with supramaximal concentration of acetylcholine (10(-5) mol/l), ryanodine (3.10(-5) mol/l) further potentiated the tone. Caffeine (10(-4)-10(-2) mol/l) decreased and even completely inhibited the tone, suggesting some other effects of caffeine. Ruthenium red concentration-dependently (10(-6)-10(-4) mol/l) decreased the tone. The present data provide evidence for the role of Ca2+ release from a SR ryanodine-sensitive Ca2+ store in the maintenance of the muscle tone of the cat gastric fundus.

Myorelaxant activity of 2-t-butyl-4-methoxyphenol (BHA) in guinea pig gastric fundus.

This study investigates the mechanism whereby the antioxidant 2-t-butyl-4-methoxyphenol (BHA) relaxes guinea pig gastric fundus smooth muscle. In circular smooth muscle strips, 10 microM cyclopiazonic acid, a specific inhibitor of sarcoplasmic reticulum Ca2+-ATPase, induced a prolonged rise in tension which depended on the presence of extracellular Ca2+. BHA (pIC50 = 5.83), sodium nitroprusside (6.85), isoproterenol (7.69) and nifedipine (8.02), but not 2,6-di-t-butyl-4-methoxyphenol (DTBHA) (up to 30 microM), relaxed muscle strips contracted with cyclopiazonic acid. Methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyri dine-5-carboxylate (Bay K 8644) (1 microM) antagonised the nifedipine- but not the BHA-induced relaxation. Nifedipine and isoproterenol (10 microM) caused a decrease in spontaneous tone, but did not counteract the subsequent rise in tension elicited by 10 microM cyclopiazonic acid. Conversely, 100 microM BHA and 100 microM sodium nitroprusside not only significantly reduced spontaneous tone but also markedly impaired the response of the muscles to cyclopiazonic acid. DTBHA failed to show either effect. When added to preparations completely relaxed by 100 microM BHA, 10 mM tetraethylammonium still elicited nifedipine-sensitive tonic and phasic contractions in the presence or absence of 10 microM cyclopiazonic acid. BHA and DTBHA inhibited, in a concentration-dependent manner, the Ca2+-promoted contraction of strips depolarised by 10 mM tetraethylammonium. The BHA antagonism showed a non-competitive profile while that of DTBHA was competitive. In muscle strips at rest, 10 microM BHA caused a significant increase in tissue cAMP concentration, leaving cGMP unmodified. To conclude, the myorelaxant action of BHA on gastric fundus smooth muscle appears to be mediated partly by an increase in cAMP levels and partly by inhibition of Ca2+ influx from the extracellular space.

Role of sarcoplasmic reticulum in the myorelaxant activity of nitric oxide donors in guinea pig gastric fundus.

The relaxant effect of two nitric oxide (NO) donors: sodium nitroprusside and 3-morpholino-sydnonimine (SIN-1) on circular smooth muscle strips isolated from guinea pig gastric fundus was studied with the view to elucidating the mechanism, which underlies the NO-induced relaxation of this tissue. Both sodium nitroprusside (10(-9)-10(-5) M) and SIN-1 (10(-9)-10(-4) M) suppressed the spontaneous fundus tone and hyperpolarized the muscle cells by about 5 mV. They antagonized the acetylcholine (10(-6) M)-induced tone and exerted their relaxant effects even when Ca2+ influx into the cells was triggered through the Na+/Ca2+ exchanger. Sodium nitroprusside and SIN-1 antagonized the contraction induced by cyclopiazonic acid (10(-5) M), a specific inhibitor of the sarcoplasmic reticulum Ca2+-ATPase. In the presence of high concentrations of sodium nitroprusside or SIN-1, cyclopiazonic acid (10(-5) M) exerted only a slight if any contractile effect. After the complete relaxation induced by sodium nitroprusside or SIN-1, the K+-channel blockers, tetraethylammonium, apamin and charybdotoxin, as well as the Ca2+ ionophore, A 23187, induced high-amplitude contractions, suggesting that the Ca2+ sensitivity of the contractile myofilaments was not affected. The results suggest that NO, released from NO donors increases the sarcoplasmic reticulum Ca2+ uptake thereby enhancing the vectorial sarcoplasmic reticulum Ca2+ release toward the plasmalemma to elicit membrane hyperpolarization and relaxation in guinea pig gastric fundus.

Two populations of smooth muscle cells in the guinea-pig gastric antrum.

K+ outward currents (I[K]) expressed by guinea-pig antral smooth muscle cells were studied using the whole-cell voltage-clamp technique. In about 88% of cells depolarization steps applied from Vh = -70 mV activated a fast transient component (I[K(to)]) with voltage-dependent characteristics, and a noninactivating component with slow activation kinetics (I[K(sl)]). Both components were carried by K+ ions. Apamin (10 nM to 1 microM) selectively depressed I(K[to]) in a concentration-dependent manner. I(K(sl)) was blocked by 1 mM tetraethylammonium or 0.1 microM charybdotoxin. 10 mM tetraethylammonium abolished both components of I(K). Nicardipine (1 microM) did not affect the voltage- and time-dependent characteristics of the net I(K), but reduced the current density of I(K[sl]) from 22.36+/-1.38 microA/cm2 to 13.06+/-0.92 microA/cm2 at +40 mV. In about 12% of the cells depolarization-evoked I(K) could be separated as two pharmacologically distinguishable components: a glipizide-sensitive current (forming about 70% of the net I[K]) and a charybdotoxin-sensitive current (30% of the net I[K]). Nicardipine (1 microM) affected neither the amplitude nor the time-course of I(K) of this cell population. The depletion of intracellular Ca2+ stores by thapsigargin (1 microM) or ryanodine (1 microM) led to a 50-200% increase of I(K[sl]) in the majority of cells and to an about 30% increase of the net I(K) in 12% of cells. The data obtained suggest the existence of at least two populations of cells in guinea-pig antral smooth muscle. Twelve percent of cells seem to be responsible for the generation of slow wave potentials, while 88% of cells most probably respond passively to the electrotonically spread depolarization.

Two components of potassium outward current in smooth muscle cells from the circular layer of human jejunum.

Two components of the outward K+ currents (Ik) of cells isolated from the circular layer of human jejunum were investigated using the conventional whole-cell voltage clamp method. A fast transient Ik component could only be elicited by depolarization in cells dialysed with pipette solution of pCa < 7.4. This Ik component was strongly voltage dependent, and could be selectively abolished by 30 mumol/l quinidine. Its amplitudes decreased in the absence of Ca entry, the decrease depending on the duration of cell exposure to media containing calcium-blockers, and disappeared after depletion of intracellular Ca2+ stores. The steady-state component of Ik was sensitive to tetraethylammonium. This component had comparable amplitudes at pCa = 8.4 or pCa = 7.4 of the pipette solution, and was present during a long-lasting exposure of cells to solutions containing Ca(2+)-blocking drugs.

Main components of voltage-sensitive K+ currents of the human colonic smooth muscle cells.

The aim of the present study is to characterize voltage-sensitive macroscopic K+ current (IK) components in freshly isolated human colonic smooth muscle cells. IK components were studied by the conventional whole-cell voltage clamp method. We found two main components of IK. A transient IK component with fast kinetics, IK(fi), activated upon a holding potential of Vh = -80 mV and inactivated completely following a 4-second-long prepulse to 0 mV. IK(fi) was abolished by Vh = -50 mV, as well as by a pipette solution containing 11 mM EGTA. The second, non-inactivating IK component, IK(ni), had comparable amplitudes at Vh = -80 mV and Vh = -50 mV and could not be inactivated completely, even by positive conditioning stimuli. The amplitudes of IK(ni) depended strongly on the Ca2+ entry, while the amplitudes and the time course of IK(fi) were modulated mainly by the intracellular Ca2+ concentration. About 80% of IK(ni) was selectively inhibited by 0.5 microM apamin. IK(fi) was insensitive to apamin and was totally abolished by 20 mM tetraethyl ammonium extracellularly. According to their voltage dependencies, inactivation properties and pharmacological sensitivity to various K+ channels antagonists, these IK components differ from those described in colonic smooth muscle cells of laboratory animals.

Cyclopiazonic acid-induced changes in contractile activity of smooth muscle strips isolated from cat and guinea-pig stomach.

The effects of cyclopiazonic acid (CPA), a specific inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, on contractile activity of circular smooth muscle strips isolated from the antrum, corpus and fundus regions of the cat and guinea-pig stomach were studied. Contractile activity was recorded under isometric conditions, in organ baths. CPA, concentration dependently (3 x 10(-7)-3 x 10(-5) M) increased the tone of the cat and guinea-pig gastric fundus and corpus as well as the amplitude of the phasic contractions of the cat corpus and antrum, affecting their frequency. CPA had a dual action on the phasic contractions of the guinea-pig antrum: an increase at low concentrations (up to 10(-6) M) and inhibition at high concentrations (10(-6)-3 x 10(-5) M). Tetrodotoxin (10(-6) M), atropine (10(-6) M) and N omega-nitro-L-arginine (10(-4) M) did not change significantly the effects of CPA. Nifedipine completely inhibited the CPA-induced phasic contractions and partly inhibited the CPA-induced tonic contractions. The nitric oxide-releasing agents, sodium nitroprusside (10(-3) M) and 3-morpholino-sydnonimine (10(-3) M), completely inhibited the CPA-induced tonic and phasic contractions. CPA induced tonic contractions in the cat and guinea-pig gastric fundus precontracted by acetylcholine (10(-5) M) and inhibited the acetylcholine (10(-6) M)-induced phasic contractions in the guinea-pig gastric antrum and corpus. The results suggest multiple roles for sarcoplasmic reticulum Ca2+ stores and sarcoplasmic reticulum Ca(2+)-ATPase in the shaping of spontaneous and evoked tonic and phasic contractions of the stomach, and highlight important species and tissue differences.

The role of sarcoplasmic reticulum and sarcoplasmic reticulum Ca2+-ATPase in the smooth muscle tone of the cat gastric fundus.

Circular smooth muscle strips isolated from cat gastric fundus were studied in order to understand whether the sarcoplasmic reticulum (SR) and SR Ca2+-ATPase could play a role in the regulation of the muscle tone. Cyclopiazonic acid (CPA), a specific inhibitor of SR Ca2+-ATPase, caused a significant and sustained increase in muscle tone, depending on the presence of extracellular Ca2+. Nifedipine and cinnarizin only partially suppressed the CPA-induced tonic contraction. Bay K 8644 antagonized the relaxant effect of nifedipine in CPA-contracted fundus. Nitric-oxide-releasing agents sodium nitroprusside and 3-morpholino-sydnonimine completely suppressed the CPA-induced tonic contraction. The blockers of Ca2+-activated K+ channels, tetraethylammonium, charybdotoxin and/or apamin, decreased the contractile effect of CPA. Vanadate increased the tone but did not change significantly the effect of CPA. CPA exerted its contractile effect even when Ca2+ influx was triggered through the Na+/Ca2+ exchanger and the other Ca2+ entry pathways were blocked. Thapsigargin, another specific SR Ca2+-ATPase inhibitor, also increased the muscle tone. The effect of thapsigargin was completely suppressed by sodium nitroprusside and 3-morpholino-sydnonimine and partially by nifedipine. In conclusion, under conditions when the SR Ca2+-ATPase is inhibited, the tissue develops a strong tonic contraction and a large part of this is mediated by Ca2+ influx presumably via nifedipine-sensitive Ca2+ channels. This study suggests the important role of SR Ca2+-ATPase in the modulation of the muscle tone and the function of SR as a "buffer barrier" to Ca2+ entry in the cat gastric fundus smooth muscle.

Ca(2+)-induced Ca2+ release activates K+ currents by a cyclic GMP-dependent mechanism in single gastric smooth muscle cells.

The participation of sarcoplasmic reticulum Ca2+ release channels in the activation of Ca(2+)-sensitive K+ currents (IK(Ca)) by cyclic dibutyryl GMP was investigated in smooth muscle cells from the circular layer of guinea-pig gastric fundus. All experiments were performed in the presence of 3 microM nicardipine into the bath and low Ca2+ buffering capacity of the pipette-filling solution (pCa 7.4). Ruthenium red (10 microM) as well as its combination with 10 microM heparin abolished the cyclic GMP-induced activation of IK(Ca), while 10 microM heparin remained ineffective. Ryanodine (10 microM) and the subsequently added 1 microM thapsigargin induced a relatively small increase in IK(Ca) amplitudes. The addition of 10 microM ryanodine to 1 microM thapsigargin-containing bath solution caused a vast increase in IK(Ca). It is hypothesyzed that protein kinase G-induced vectorial Ca2+ flux from the cell bulk and sarcoplasmic reticulum Ca2+ stores toward the plasma membrane is realized by a spontaneous Ca(2+)-induced Ca2+ release from a superficially situated Ca2+ store.

Three components of potassium outward current in cells isolated from the circular layer of guinea-pig ileum.

Three different populations of voltage-activated K+ channels were identified according to their voltage-dependences, inactivation kinetics and sensitivity toward selective K+ channels antagonists in membranes of cells, isolated from the circular layer of guinea-pig ileum by the use of whole-cell voltage clamp method. Under holding potential Vh = -50 mV apamin (10-6 mol/l) inhibited 29 +/- 3% of the whole-cell K+ current (IK) at 0 mV membrane potential. The apamin-blockade component of IK disappeared after blockade of the inward Ca2+ current, while other IK components remained unchanged. Glibenclamide (10-5 mol/l) inhibited 31 +/- 2.6% of IK at 0 mV membrane potential. Cromakalim (5.10-7 mol/l) increased IK, and glibenclamide impeded this increase and reduced IK amplitudes to the same extent as when applied alone. The difference between amplitudes of IK, measured from Vh = -80 mV and the above ones, expressed from Vh = -50 mV, was defined as the potential-dependent part of IK (Kpd). This component formed 36 to 47% of IK in cells clamped at Vh = -80 mV, and was inhibited by charybdotoxin (10-6 mol/l). The physiological role of these components of IK is discussed.

Cyclic GMP-induced activation of potassium currents by sarcoplasmic reticulum Ca2+ pump-dependent mechanism.

In voltage-clamped single smooth muscle cells from the circular layer of the guinea-pig gastric fundus NO-liberating substance or an analogue of cyclic 3, 5'-guanosine monophosphate (cGMP) increased or decreased the outward K+ current amplitudes depending on the Ca2+ buffering capacity of the intracellular medium. In a high EGTA-containing pipette solution dibutyryl-cGMP or sodium nitroprusside (SNP) attenuated both the fast and the late K+ current components. In pipette solution with lower Ca2+ -buffering capacity these drugs caused a sustained increase of K+ current amplitudes, which was effectively antagonized by thapsigargin, an inhibitor of Ca2+ -ATPase in the sarcoplasmic reticulum (SR). Our data suggest that, in gastric fundus smooth muscles, NO-liberating substances and cGMP analogues contribute to the activation of a Ca2+ -release mechanism from the cell bulk, i.e. the myoplasm surrounding the contractile filaments, towards the plasma membrane, crossing the SR Ca2+ -stores. Thus, a decreased intracellular free calcium concentration ([Ca2+]) is coupled with an elevation of subplasmalemmal calcium, which in turn causes cell membrane hyperpolarization. The latter is a consequence of the opening of tetraethylammonium-sensitive Ca2+ -activated K+ channels and leads to sustained smooth muscle relaxation, most characteristic for gastric fundus preparations.

L-type calcium channels may fill directly the IP3-sensitive calcium store.

The relationship between inositol-1,4,5-trisphosphate-sensitive Ca2+ stores and Ca2+ entry through potential-dependent L-type Ca2+ channels was examined using whole-cell voltage-clamp technique in cells of longitudinal muscle layer of guinea-pig ileum. It was found that heparin (10(-10) mol/l) in the pipette rapidly inhibited the current through L-type Ca2+ channels. Neither an inhibitor of the sarcoplasmic reticulum Ca2+ pump (cyclopiazonic acid) nor blockers of Ca(2+)-induced Ca2+ release (ryanodine or ruthenium red) affected the Ca2+ current. The failure of heparin to affect Ca(2+)-currents through L-type Ca2+ channels in cells from circular muscle of the same organ suggested that heparin had no direct effect on L-type Ca2+ channels. Thus the inhibition of the latter in heparin-loaded cells from the longitudinal layer is supposed to be Ca(2+)-dependent due to the overfilling of the inositol-1,4,5-trisphosphate-sensitive Ca2+ store.

Inhibition of Ca2+ current in ileal cells by cyclopiazonic acid and ryanodine.

The effects of cyclopiazonic acid and its combination with ryanodine on the inward Ca2+ current (ICa) were investigated in smooth muscle cells isolated from the circular layer of guinea-pig ileum. The ICa of these cells exhibited two components: a low-threshold, nicardipine (5 microM)-resistant, fast-inactivating component and a high-threshold, nicardipine-blockable and slowly inactivating component. Neither cyclopiazonic acid (up to 10 microM) nor ryanodine (10 microM) was able to affect both these components of ICa, when applied separately. Cyclopiazonic acid and ryanodine combination led to total abolishment of the high-threshold component, leaving the low-threshold component unaffected. The data presented suggest a process of Ca(2+)-dependent inactivation of the high-threshold component, elicited by an increase in the subplasmalemmal Ca2+ concentration due to Ca2+ released from the sarcoplasmic reticulum. It is considered that the combination of cyclopiazonic acid and ryanodine can be used as a valuable method to study the calcium sensitivity of both components of the ICa.

Two types of Ca(2+)-channels in cells from the circular layer of guinea-pig ileum.

In this paper data are presented concerning some morphometrical characteristics and passive electrical properties of cell membranes from the circular layer of guinea-pig ileum (CC). The main purpose of the study has been to analyze the inward calcium current in these cells. Using the whole-cell voltage-clamp method the presence of T-like type of calcium channels (conducting low-threshold, fast inactivated, strictly potential-dependent and dihydropyridine-resistant calcium inward current--If) could be found in membranes of the cells investigated. The participation of If in the total inward current of CC was deduced from the presence of a low-threshold "hump" in the current-voltage relation curve (I/V-curve) between -80 and -50 mV, and also from a negative shift of this curve at holding potential Vh = -90 mV, compared to the one obtained at Vh = -50 mV. Besides, in the presence of a dihydropyridine Ca2+ antagonist, a fast inactivating component of the inward current could be recorded, which was effectively blocked by 0.3 mmol/l Ni2+ from outside. According to Tsien's criteria (1983) our data suggest the coexistence of T- and L-types calcium channels in CC.

Participation of calcium, released from the IP3-sensitive Ca-store in activation of Ca-dependent potassium conductance of ileal smooth muscle cells.

The apamin-, charybdotoxin- (CTX) and glibenclamide- (GLB) sensitive components, which form the active part of the net potassium outward current (IK) in single smooth muscle cells from the longitudinal layer of guinea-pig ileum (LC), were investigated for their sensitivity to calcium. The experiments were carried out by the whole-cell voltage-clamp method. A successful block of all Ca-sources (with heparin and nifedipine; heparin and cyclopiazonic acid while the intracellular Ca-concentration--[Ca2+]i--was kept at 3 x 10(-8) mol/l by 11 mmol/l EGTA into the pipette solution) led to the complete inhibition of IK. The deeply located Ca-sensitive Ca-pool was effectively isolated by the high concentration of the chelator, which was proved by the fact that ruthenium red and ryanodine failed to affect IK. The GLB-sensitive component of IK demonstrated Ca-gated properties, while both the other components were activated most probably by calcium, released form the IP3-sensitive Ca-pool. It was concluded that the IP3-induced Ca-release mechanism plays an important role in the regulation of K(+)-conductivity in LC.

Electro-mechanical coupling in the complex stomach smooth muscles.

In the majority of experiments the ruminal preparations did not exhibit spontaneous electrical and contractile activity but showed a Ca-dependent spike-free myogenic tone. Ba2+ or Sr2+ evoked spike activity and phasic contractions and increased the muscle tonic tension. Tetraethylammonium (TEA) exerted no effect on the ruminal preparations but potentiated the effect of Ba2+. Acetylcholine (ACh) induced dose-dependent spike-free tonic contractions which were verapamil resistant and were suppressed by sodium nitroprusside. When applied to BaCl2- or TEA-pretreated preparations ACh caused spike activity and phasic contractions. The antral abomasum exhibited a spontaneous electrical activity--slow waves on which spike potentials were superimposed. ACh and TEA induced membrane depolarization and increased the spike activity and the phasic contraction amplitude.