J Vandenakker

The action of apamin on guinea-pig taenia caeci

Apamin (10(-7) M), a substance extracted from bee venom (apis mellifica) causes stimulation of the taenia caeci as seen from an increase in spike activity. The inhibitory effect of ATP or adrenaline (Adr) was reflected by hyperpolarization of the muscle cell, cessation of spike activity and relaxation of the muscle. The 42K efflux and the membrane conductance were enhanced in the presence of these substances. Apamin converted the hyperpolarization caused by ATP or Adr into a transient depolarization which produced contraction of the muscle cells. The changes in membrane conductance and 42K efflux were diminished by the bee toxin. Furthermore, the potassium-dependent phase of the action potential was lengthened by apamin. Reduction of the extracellular chloride or sodium concentration, blockade of the nervous system by TTX (3 x 10(-7) M) or inhibition of spike activity by D600 (3 x 19(-6) M) did not affect the excitatory and blocking action of apamin. A high concentration of the calcium antagonist D600 (10(-4) M) or omission of extracellular calcium was needed to reduce the transient depolarization evoked by ATP or Adr in the presence of apamin. It is concluded that apamin prevents the opening of the ATP- and Adr-sensitive and voltage-dependent potassium channels in guinea-pig taenia caeci.

The inhibitory action of suramin on the P2-purinoceptor response in smooth muscle cells of guinea-pig taenia caeci

The effect of suramin on the smooth muscle cell response of guinea-pig taenia caeci to P2-purinoceptor and alpha 1-adrenoceptor stimulation was measured. The ATP-induced relaxation in potassium (20 mM) pre-contracted taenia caeci was inhibited by suramin (3 x 10(-4) M). The P2-purinoceptor-induced hyperpolarization elicited by ATP both in the presence and absence of calcium was also reduced by suramin. The alpha 1-adrenoceptor-mediated relaxation evoked by phenylephrine was only affected by suramin at low concentrations of the agonist. The results indicate that suramin inhibits the ATP response by interacting with P2y-purinoceptors.

The contribution of calcium and potassium to the α-action of adrenaline on smooth muscle cells of the portal vein, pulmonary artery and taenia caeci of the guinea-pig

The role of calcium and potassium in the alpha-action of adrenaline in pulmonary artery and portal vein was compared with that in taenia caeci by measuring changes in membrane potential, muscle contraction and ion fluxes in quiescent preparations from guinea-pigs (23 degrees C). The depolarization evoked by adrenaline (5 x 10(-8)-3 x 10(-5) M) was sustained in portal vein; in pulmonary artery it declined to a constant level after reaching an initial maximum. In calcium-free medium (20 min) containing EGTA (0.4 mM) and high magnesium (6.2 mM) adrenaline did not affect the membrane potential or the contractile state of the portal vein. Under these conditions the sustained phase of the response was abolished in the pulmonary artery; the remaining transient depolarization and contraction could be evoked only once. Adrenaline (3 x 10(-5) M) caused an increased 45Ca loss and 86Rb loss from the pulmonary artery and taenia caeci in calcium-free solution; a second addition of adrenaline to the calcium-free solution did not enhance the 45Ca loss from these tissues. The portal vein responded with an enhanced 86Rb loss on addition of the alpha-agonist. The bee toxin apamin (3 x 10(7) M) did not modify the depolarization, the contraction or the 45Ca and 86Rb fluxes evoked by adrenaline in the blood vessels. Enhancement of the 86Rb loss from taenia in the presence of adrenaline was prevented by apamin, but the excess loss of 45Ca was not abolished. It is concluded that adrenaline enhances cytoplasmic calcium by promoting calcium entry from the extracellular space in portal vein. In pulmonary artery and taenia caeci this is accompanied by mobilization of calcium from a cellular structure. Calcium entry facilitates triggering of the contractile proteins in vascular smooth muscle and is associated with membrane depolarization; in taenia caeci the mobilization of calcium caused by alpha-receptor activation is associated with the opening of potassium channels producing hyperpolarization and accordingly relaxation of the smooth muscle cells.

The action of prostaglandin E2 on the smooth muscle cell of the guinea-pig taenia coli

Abstract The effect of prostaglandin E 2 (PGE 2 ) on the smooth muscle cell of guinea pig taenia coli was investigated by measuring changes in membrane potential, membrane resistance, muscle contraction and ion fluxes. The action of PGE 2 caused depolarization of the cell membrane, and increased spike activity and muscle contraction. The amplitude of the PGE 2 depolarization was concentration-dependent. The action of PGE 2 could be mimicked by depolarizing the muscle membrane with an external current with as result a decrease in membrane resistance similar to that observed in the presence of PGE 2 . The PGE 2 depolarization was not abolished when the sodium pump was inhibited by omitting external potassium or in the presence of K-strophantin, persisted in low chloride Krebs and in low sodium solution, but was reduced in calcium-free solution and in the presence of D600. 45 Ca influx was enhanced by PGE 2 , but 45 Ca efflux was not affected. The 42 K efflux was increased in the presence of PGE 2 , but this response was blocked by Ca-free solution. These results indicate that calcium permeability is enhanced by PGE 2 , and that this enhancement is associated with a depolarization of the muscle cell membrane of the guinea-pig taenia coli.

Modification of α1-receptor-operated channels by mebeverine in smooth muscle cells of guinea-pig taenia caeci

Abstract Changes in the potential and contractility of smooth muscle cells of guinea-pig taenia caeci were measured (22°C) in order to investigate the effect of mebeverine, a derivative of β-phenylethylamine, on α 1 -receptor-operated ion channels in particular. Mebeverine (6×10 −6 M showed atropine-like properties by shifting to the right the concentration-response curve obtained with carbachol. Hyperpolarization and cessation of spike activity of the muscle cells, accompanied by an increase amplitude of the electronic potential were observed in the presence of mebeverine (6 × 10 −5 ) after block of the α 2 -, β- and muscarinic receptors. This effect of mebeverine was not observed in low-sodium solution (23.8 mM), suggesting that mebeverine decreased sodium permeability. The α 1 -receptor-induced hyperpolarization caused by adrenaline (3 × 10 −6 M) in the presence of mebeverine declined after reaching an initial maximum. The hyperpolarization induced by a second addition of adrenaline to the preparation was decreased and sustained in the presence of mebeverine, while the decrease of the electronic potential evoked during the α 1 response was less pronounced. The transient hyperpolarization representing the α 1 response in the absence of extracellular calcium developed more slowly in the presence of mebeverine, the area of the response being constant. When the experiment was continued in calcium-free solution after a short exposure to calcium-containing Krebs solution still in the presence of mebeverine, the α 1 -receptor-induced hyperpolarization was suppressed. Depolarization of the muscle cells was observed on addition of adrenaline to the preparation in calcium-free conditions and in the presence of apamin (3 × 10 −7 M). Mebeverine affected this reversed α 1 response similarly to the hyperpolarization observed without apamin. The results show that mebeverine inhibited the α 1 action by limiting the amount of available calcium mobilized on receptor stimulation of the smooth muscle cells of guinea-pig taenia caeci.

EFFECTOR MECHANISMS FOR ALPHA,BETA-METHYLENE ATP AND ATP DERIVATIVES IN GUINEA-PIG TAENIA-CAECI

The effects of the ATP analog alpha, beta-methylene ATP and ATP derivatives on smooth muscle cells of guinea-pig taenia caeci were studied by measuring potential changes. The preparations were superfused (1 ml/min) with Krebs solution or calcium-free solution containing atropine (10(-6) M), prazosin (10(-6) M) and propanolol (10(-6) M) using the sucrose-gap method at 22 degrees C. The ATP analog (10(-5) M to 4 X 10(-4) M) caused slowly developing hyperpolarization in the presence of calcium. Hyperpolarization also occurred on addition of the analog in calcium-free medium. The area of the response increased with the analog concentration (10(-5) M to 4 X 10(-4) M). The response evoked after repeated addition of the ATP analog had slightly lower amplitude and slower onset than the first response both in the presence and the absence of calcium. The area of the response to the ATP analog in the absence of calcium was also decreased when preceded by the ATP response. When the opposite experimental sequence was used the ATP response was strongly inhibited, when preceded by the analog response evoked by the same concentration. The ATP analog caused depolarization of the muscle cells in the presence of apamin (3 X 10(-7) M), which was also observed with ATP. The ATP derivatives (4 X 10(-4) M) caused hyperpolarization of the smooth muscle cell membrane, except hypoxanthine and inosine which were inactive.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of cromakalim on smooth muscle cells of guinea-pig taenia caeci.

Cromakalim caused hyperpolarization and reduction of the electrotonic potential in a concentration-dependent manner in smooth muscle cells of guinea-pig taenia caeci. There was a relatively constant change in the electrotonic potential under calcium-free, low-sodium and low-chloride conditions in the presence of cromakalim as compared to control conditions with Krebs solutiuon. The effect of cromakalin (10(-5) M) was inhibited by glibenclamide (5 X 10(-5) M). These results indicate that cromakalim specifically promotes potassium efflux in smooth muscle cells of the guinea-pig taenia caeci via glibenclamide-sensitive potassium channels, to cause hyperpolarization, suppression of spike activity and relaxation.

The action of mebeverine and metabolites on mammalian non-myelinated nerve fibres

The effect of mebeverine, mebeverine-alcohol and veratric acid on voltage-operated ion channels and on sodium pump activity were studied. The amplitude and duration of the compound action potential evoked in the vagus nerve were decreased by mebeverine but not by the metabolites. The amplitude of the post-tetanic hyperpolarization was decreased by mebeverine while the time constant of decay, representing sodium-potassium pump activity, was not affected by the compounds tested. These results show that mebeverine exerts a local anaesthetic action by blocking voltage-operated sodium channels.

THE INFLUENCE OF ALPHA-BETA-METHYLENE ATP ON ALPHA-1-RECEPTOR-OPERATED CHANNELS IN GUINEA-PIG TAENIA CECI

Abstract The influence of the ATP analog α,β-methylene ATP on the action of adrenaline on α 1 -receptors of smooth muscle cells of guinea-pig taenia caeci was studied by measuring potential changes. The preparation was superfused (1 ml/min) with Krebs solution or calcium-free solution containing atropine (10 −6 M) and propranolol (10 −6 M) at 22°C, using the sucrose-gap method. The ATP analog (10 −5 to 4 × 10 −4 M) and adrenaline (10 −5 M) both caused a transient hyperpolarization in the absence of external calcium (20 min). The response (area under the ‘curve’) evoked under calcium-free conditions (20 min) increased with the concentration of the ATP analog. The response was diminished when preceded by the adrenaline response (10 −5 M) or when evoked after repeated addition of the analog to the superfusate (35 min). The adrenaline response was also diminished when preceded by the ATP analog. The responses to the ATP analog or adrenaline in the presence of apamin (3 × 10 −7 M) in the absence of external calcium were characterized by depolarization of the muscle cells. Repeated addition of the ATP analog or adrenaline to the preparation under these conditions did not cause any effect. The results suggest strongly that adrenaline and α,β-methylene ATP both activate the same calcium-dependent process, producing calcium mobilization and the opening of apamin-sensitive potassium channels. Besides this action the ATP analog also activates apamin-sensitive potassium channels and this activation is independent of the availability of calcium in the adrenaline-sensitive pool.Abstract The influence of the ATP analog α,β-methylene ATP on the action of adrenaline on α 1 -receptors of smooth muscle cells of guinea-pig taenia caeci was studied by measuring potential changes. The preparation was superfused (1 ml/min) with Krebs solution or calcium-free solution containing atropine (10 −6 M) and propranolol (10 −6 M) at 22°C, using the sucrose-gap method. The ATP analog (10 −5 to 4 × 10 −4 M) and adrenaline (10 −5 M) both caused a transient hyperpolarization in the absence of external calcium (20 min). The response (area under the ‘curve’) evoked under calcium-free conditions (20 min) increased with the concentration of the ATP analog. The response was diminished when preceded by the adrenaline response (10 −5 M) or when evoked after repeated addition of the analog to the superfusate (35 min). The adrenaline response was also diminished when preceded by the ATP analog. The responses to the ATP analog or adrenaline in the presence of apamin (3 × 10 −7 M) in the absence of external calcium were characterized by depolarization of the muscle cells. Repeated addition of the ATP analog or adrenaline to the preparation under these conditions did not cause any effect. The results suggest strongly that adrenaline and α,β-methylene ATP both activate the same calcium-dependent process, producing calcium mobilization and the opening of apamin-sensitive potassium channels. Besides this action the ATP analog also activates apamin-sensitive potassium channels and this activation is independent of the availability of calcium in the adrenaline-sensitive pool.

The action of some new aminopyridines on mammalian non-myelinated nerve fibres.

The effects of a recently synthesized series of aminopyridines 2-methyl-4-AP, 2-chloro-4-AP and 2-(N,N-methyl-benzyl)amino-4-AP (2A-7) on voltage-operated sodium and potassium channels and on the sodium pump activity of non-myelinated fibres of the guinea-pig vagus nerve were studied with the sucrose-gap method. The compound action potential evoked by electrical stimulation and the propagation velocity along the nerve were not affected by 2-methyl-4-AP or 2-chloro-4-AP up to a concentration of 10(-3) M. The post-tetanic potential (PTH) evoked by repetitive stimulation of the nerve and reflecting sodium pumping was also not affected by these agents. The amplitude and duration of the compound action potential were enhanced to some extent by 2-methyl-4-AP at the highest concentration used (3 X 10(-3) M); this action was also observed and was more pronounced with 4-aminopyridine (4-AP). The other aminopyridine 2A-7 (3 X 10(-5) - 3 X 10(-4) M) caused suppression of the compound action potential, a diminished propagation velocity and a reduction of the PTH, an action also observed with lidocaine. These results show that 2-methyl-4-AP and 3-chloro-4-AP did not affect the voltage-operated sodium or potassium channels in non-myelinated fibres of the vagus nerve. Only 2-methyl-4-AP had a small 4-AP-like action at high concentrations. The aminopyridine 2A-7 possesses a local anaesthetic action as reflected by the inhibition of voltage-operated sodium channels.