Membrane potentials recorded with high-resistance micro-electrodes; and the effects of changes in ionic environment on the electrical and mechanical activity of the smooth muscle of the taenia coli of the guineapig

Patch-clamp studies of slow potential-sensitive potassium channels in longitudinal smooth muscle cells of rabbit jejunum

1. The patch-clamp technique was used to study single channel currents in membrane patches of longitudinal smooth muscle cells of rabbit jejunum dispersed by collagenase treatment. Recordings were made from both cell-attached and isolated patches.2. The predominant unit currents observed were outward at membrane potentials positive to the potassium equilibrium potential (E(K)) and they were rapidly and reversibly blocked by tetraethylammonium (TEA). Their size varied as E(K) was changed but was not noticeably affected by changing E(Na), E(Cl) or E(Ca); it was little altered in calcium-free EGTA solution. Thus, these currents apparently result mainly, if not exclusively, from the movements of potassium ions through channels insensitive to the calcium ion concentration. The present study describes the properties of these potassium channels.3. The unit conductance varied slightly with potential in most experiments; around zero potential it was about 50 pS. The conductance was dependent upon the potassium, but not the calcium, gradient. Sub levels of conductance of about two-thirds and, less commonly, one-third of the fully conducting channel state were sometimes seen.4. Membrane patches were studied which showed one to about twelve levels of outward current which were presumed to result from the opening of up to twelve channels having the same characteristics. The probability of channel open state varied with membrane potential, increasing in the potential range -40 to +40 mV. Channel openings were rare negative to -40 mV. No inward currents through these potassium channels were observed as openings were not seen at membrane potentials negative to E(K).5. When the probability of channel opening was low, channel openings occurred in bursts which could be separated by several seconds. Analysis of the openings of a single channel revealed that open times and short closed times were exponentially distributed with mean durations of 15-45 ms and about 6 ms at zero potential. In some patches regular cyclical openings of several channels occurred. In other patches openings of individual channels appeared to be independent events as they were reasonably fitted by a binomial distribution.6. Following a step change from negative potentials, where channels were closed, to more positive potentials, channel openings increased during a period of 10 s to reach a steady state. No evidence of inactivation was observed.7. These results suggest the existence of a population of potential-sensitive potassium-selective ion channels in the smooth muscle cell membrane which are closed at the resting membrane potential and which open upon depolarization with slow (seconds) kinetics; these may be involved in the slow potential (wave) activity of this muscle.

Towards an estimate of chloride permeability in the smooth muscle of guinea-pig vas deferens

Cl movements across the cell membranes of smooth muscle from the guinea-pig vas deferens were measured using Cl-sensitive micro-electrodes and 36Cl fluxes. The rate constants for the loss of Cl ions measured by both methods under a variety of conditions were used to calculate the apparent Cl permeability (PCl). If it is assumed that the initial rate of decline of the intracellular Cl activity (aiCl) on removal of extracellular Cl (Clo) represents net transmembrane Cl movement, the apparent PCl was 3-6 X 10(-8) cm s-1. This value is in good agreement with those calculated from the rate constant of 36Cl efflux into both normal Krebs solution (steady-state) and Cl-free solution. Such a value for PCl predicts a large depolarization on removal of Clo, but only a minimal change was recorded. It also predicts that changes in membrane potential (Em) would affect aiCl; furthermore that removal of Clo would increase membrane resistance and thus the hyperpolarization observed on reactivation of the electrogenic Na pump. Neither of these was observed. The PCl/PK ratio obtained from changes in Em on rapid changes in Clo and Ko gives a value for PCl which is an order of magnitude lower: 4 X 10(-9) cm s-1, using Casteels' (1969 b) value for PK. These observations can be reconciled by a substantial proportion of the measured Cl movements being carrier-mediated. The presence of the stilbene derivative DIDS greatly slowed both the steady-state efflux and uptake of 36Cl, as has previously been shown for the loss and reaccumulation of Cl ions on removal and replacement of Clo. PCl calculated in the presence of DIDS was about 5 X 10(-9) cm s-1. The nominal absence of CO2 and HCO3, which slows the reaccumulation and loss of Cl, had no effect on the steady-state fluxes. This indicates that the carrier operates in the self-exchange mode in the steady state.

Ionic basis of the resting potential of submucosal arterioles in the ileum of the guinea-pig

1. The changes in the resting membrane potential of arterioles produced by rapid and brief changes in external ionic concentrations were measured. 2. The resting membrane potential was insensitive to changes in the external concentrations of both sodium and chloride ions but sensitive to changes in the external concentration of potassium ions. 3. Increasing the external concentrations of potassium ions produced depolarizations that were well described by the Nernst equation. 4. Decreased external concentrations of potassium ions produced membrane depolarizations which appeared to result not from inhibition of an electrogenic sodium pump but rather from a change in the resting conductance of the arteriolar membrane to potassium ions. 5. Ouabain caused both membrane depolarization and an increase in membrane resistance. 6. It is suggested that at rest, arteriolar smooth muscle is permeant predominantly to potassium ions, with only small contributions from chloride and sodium ions. No evidence was obtained which would support the idea that an appreciable proportion of the resting membrane potential depended upon current flow from an electrogenic sodium pump.

The calcium antagonistic effects of cyproheptadine on contraction, membrane electrical events and calcium influx in the guinea-pig taenia coli

1 The ability of cyproheptadine (Cph) to inhibit membrane translocation of calcium in smooth muscle was investigated by studying the drug's action on contraction, electrical activity and calcium influx in the guinea-pig taenia coli.2 Cph >/= 10(-6)M reduced the amplitude of normal spontaneous contractions and concurrently decreased the number of action potentials occurring with each slow-wave of depolarization (sucrose-gap recordings). These inhibitory effects of Cph were antagonized by increasing the medium [Ca] three fold to 7.68 mM.3 Intracellular recordings showed that Cph >/= 2 x 10(-6)M decreased the amplitude and extended the duration of the action potential. These effects were only partially reversible in normal medium whereas large overshooting action potentials were again seen in 7.68 mM Ca medium.4 High frequency mechanical activity was produced by inclusion of veratridine 5 x 10(-6)M in the perfusate. Low concentrations of Cph (>/= 10(-7)M) reduced the amplitude of such contractions at a faster rate than they did normal spontaneous contractions.5 At concentrations between 10(-7) and 10(-6)M, Cph fully reduced the tonic component of contractions elicited in 112 mM isotonic KCl whilst having little or no effect on either (i) the initial phasic KCl contraction or (ii) the ;repolarization contracture' normally produced on wash-out of the KCl or (iii) the spontaneous contractions before and after KCl treatment. In contrast, at Cph 2 x 10(-6)M, the repolarization contracture, as well as the isotonic KCl contraction, was totally blocked whereas spontaneous contractions were still unaffected. Progressively higher Cph concentrations inhibited all components of this contractile cycle.6 Dose-response curves for the rate of drug-induced relaxation of tonic contractures produced in hypertonic 42.7 mM high-potassium medium, showed the calcium antagonistic potency of Cph to be intermediate between that of chlorpromazine and D600. The minimum Cph concentration for effect lay between 1 and 5 x 10(-7)M, and the effects of Cph 2 x 10(-6)M (approximately the ID(50)) were totally antagonized by 12.8 mM Ca.7 By means of a lanthanum wash procedure, Cph >/= 2 x 10(-6)M was found to decrease the (45)Ca uptake occurring into strips of taenia coli in normal medium, although the maximum effect (at Cph 10(-5)M) amounted to only 25% inhibition of the uptake occurring into control strips (also found with D600). The increased uptake occurring in hypertonic 44.7 mM high-potassium medium was inhibited in a dose-dependent manner by Cph 1 x 10(-7)M.8 The results are consistent with an action of Cph in reducing the flow of Ca(2+) through voltage-dependent Ca channels in the smooth muscle cell membrane. It is suggested that the interaction of Cph molecules with such sites is dependent upon membrane potential as well as drug concentration.

Secoverine hydrochloride is a muscarinic antagonist in human isolated gastrointestinal muscle and myometrium

Secoverine is a new antimuscarinic agent thought to be selective for smooth muscle. We have found it to be a potent anticholinergic antagonist of acetylcholine in strips of human gastrointestinal and uterine muscle. The pA2 values in gut longitudinal and circular muscle and myometrium were 9·1, 9·3 and 8·6 respectively.

Ionic dependence of electrical activity in small mesenteric arteries of guinea-pigs

The regulation of blood vessel diameter is under the control of the autonomic nervous system (as well as hormones and metabolites), sympathetic nerve stimulation evoking depolarizing post-synaptic potentials. Excitatory junction potentials (EJPs) were recorded from vascular smooth muscle cells of guinea-pig small mesenteric arteries (pressurized) following nerve stimulation. Repetitive stimulation (greater than 5Hz) led to summation of EJPs, which evoked spikes and vasoconstriction. Replacing extracellular Na+ with choline (plus atropine) resulted in a decrease in EJP amplitude, but spike amplitude and maximum rate of rise (+Vmax) were unaffected. Decreasing the Ca2+ concentration produced decreases in EJP amplitude and spike +Vmax. Verapamil and bepridil, agents that depress Ca2+ influx in vascular and visceral smooth muscle, depolarized the membrane and depressed EJPs and spikes at higher concentrations (10(-5) M and 5 X 10(-6) M, respectively). The data indicate that EJPs are dependent on external Na+ and Ca2+ ions, and that spikes are dependent on Ca2+. Thus, neuromuscular transmission in this muscle is similar to that in non-vascular smooth muscles, such as intestinal muscles and vas deferens.

Membrane potentials in an acanthocephalan worm (macracanthorhynchus hirudinaceus)

The resting membrane potential of the acanthocephalan rete system in Macracanthorhynchus hirudinaceus was -35 +/- 1.5 mV (n = 20) and was dependent upon the external potassium concentration. The membrane potential reached 0 mV when the external potassium concentration was 160 mM. Spontaneous spike potentials of 45 mV +/- 10 were dependent on calcium flux. The membrane potential was depolarized by acetylcholine, potassium-free medium, calcium ions and chloride-free medium but not by changes in the external sodium concentration. Spontaneous potentials were increased in number by acetylcholine and calcium at concentrations above 3 mM, but were decreased in number by chloride- and calcium-free medium. Hence the rete system potentials are very similar to smooth muscle potentials in many respects.

Effects of histamine, high potassium and carbachol on 42K efflux from longitudinal muscle of guinea-pig intestine

1. Shortening and rate of loss of (42)K were studied in strips of longitudinal muscle taken from guinea-pig ileum.2. Carbachol, histamine and raising the external potassium concentration, [K(+)](o), to 120 mm in the presence of atropine caused equal maximal shortenings of the muscle, but unequal maximal increases in (42)K efflux: maximal (42)K effluxes obtainable in response to raised [K(+)](o) and histamine were about (2/3) and (1/3) respectively of the maximal efflux in response to carbachol. In the absence of atropine the increase in (42)K efflux produced by 120 mm-[K(+)](o) was about 50% larger, probably because of the release of acetylcholine from nerve endings in the tissue.3. If inhibitors of histamine metabolism were applied, or a H(2)-receptor blocker (cimetidine), the maximum (42)K efflux produced by histamine was not increased. An analogue of histamine reputed to resist metabolic degradation did not produce a larger increase in (42)K efflux than histamine. The smaller maximal effect of histamine on (42)K efflux than carbachol may be because it can open fewer ion channels in the smooth muscle membrane.4. The ratio of the concentrations producing 50% maximal shortening and 50% maximal (42)K efflux was about 1:1.3 for raised [K(+)](o) but about 1:20 for histamine and carbachol. Depolarization by raising [K(+)](o) appears to be less effective in causing tension development than similar depolarizations produced by carbachol or histamine.5. The relative effects of carbachol, histamine and raised [K(+)](o) were discussed in the light of their similar depolarizing actions. Increases in (42)K efflux did not appear to be caused primarily either by contraction or by depolarization of the muscle. Access of the stimulant to cells and receptors other than those which are superficially situated was suggested as being an important factor in deciding the smaller increase in (42)K efflux seen with some stimulants. Histamine receptors may be fewer in number than muscarinic receptors and less able in their activated form to open channels through which potassium ions can escape.

Effects of sodium gradient manipulation upon cellular calcium, 45Ca fluxes and cellular sodium in the guinea-pig taenia coli

1. Sucrose and choline were utilized as NaCl substitutes in order to investigate Na-Ca interactions in the smooth muscle of the guinea-pig taenia coli.2. Progressive substitution of NaCl by sucrose caused a progressive increase in cellular exchangeable Ca. This uptake, which amounted to about 300 mumole Ca/kg tissue upon total Na replacement, reached a plateau within 20 min. Complete substitution of NaCl by choline chloride caused cellular Ca to increase rapidly to an initial peak, and then decrease to a stable plateau which was also about 300 mumole/kg above control.3. Replacement of NaCl by either sucrose or choline chloride caused a transient increase in the Ca influx rate, which was measured using a 3 min pulse labelling with (45)Ca. This increase was more pronounced in choline chloride.4. NaCl substitution by either sucrose or choline chloride caused a decrease in the (45)Ca efflux rate. Two exponential components of transmembrane (45)Ca efflux were found in control and Na-free media.5. Treatment of tissues with 3 x 10(-5)m-ouabain did not significantly affect the cellular Ca content after 80 min, at which time the Na and K gradients were largely dissipated.6. Removal of medium K caused a slower dissipation of the Na and K gradients. This treatment decreased cellular Ca, did not affect the Ca influx rate, and increased the (45)Ca efflux rate.7. Tissues were incubated in depolarizing media containing 10(-4)m-ouabain in order to remove the Na gradient. Subsequent measurement of cellular Na indicated the absence of a significant fraction of bound Na.8. The ratio [Na](o)/[Na](i) had a value of 6.3 in control medium, and decreased as [Na](o) was progressively lowered by sucrose substitution, reaching a value of < 1 in a medium containing 5 mm-Na.9. These experiments provide evidence that a Na-Ca exchange carrier does not play an important role in regulation of tension in this muscle, and also indicate that the Ca gradient is not solely dependent on the Na gradient in guinea-pig taenia coli.

The role of calcium and cyclic AMP in the contractile action of angiotensin II upon rat descending colon

The effect of procedures which affect calcium availability and tissue levels of cyclic AMP were investigated upon the contractions of rat colon to angiotensin II, PGE2 and KCl. Calcium free Tyrode containing EDTA (25 microM) reduced the response to angiotensin more than to PGE2 and KCl. An increase in calcium concentration to 3.6 mM potentiated responses to angiotensin and KCl. A further increase to either 7.2 mM or 10.8 mM still potentiated responses to angiotensin but those to KCl wre reduced. SKF 525A (2.6 x 10(-5) M) and verapamil (3.5 x 10(-6) M) inhibited the responses to KCl but the responses to PGE2 and angiotensin were less affected. Isoprenaline, Theophylline and dibutyryl cyclic AMP preferentially reduced the responses to angiotensin and PGE2. It is suggested that part of the response to angiotensin involves an influx of calcium, independent of depolarisation.

Passive stress relaxation followed by active contracture in vertebrate smooth muscles (taenia coli of the guinea pig)

1. The time course of stress relaxation following stretch was investigated in the taenia coli of the guinea pig in the relaxed (Ca2+ free bath solution with D 600) and in the contracted state (depolarization by KCl or K2SO4 in excess). The mechanical tension was standardized with respect to the volume of the samples. 2. The tension obtained by a constant stretch of 2.5 mm of the 5-15 mm samples was highest after K2SO4-depolarization (K+ = 180 mval/l) and lowest in the relaxed muscles. Muscles contracted by KCl in escess (K+ = 60 mval/l) showed intermediate values. 3. The decrease of tension by stress relaxation to a nearly constant residual value is rapidly observed in contracted muscles during about 10 s. For the relaxed muscles the same changes are slower and take place in nearly 1/2 h. 4. The relations between the extent of stress relaxation (R) and its derivative in time (dR/dt) can be expressed by a hyperbolic function. Analogous behaviour was already noted in previous studies primarily on creep. 5. The different behaviour of the muscle in the different bath solutions used is discussed with respect to the mathematical relations just mentioned. 6. Long term active reactions of depolarized muscles following stretch and stress relaxation are noted.

Comparative studies of purinergic nerves

Purinergic nerves supply the gastrointestinal tract of vertebrates, including fish, amphibians, reptiles and birds, as well as mammals. Their cell bodies are located in Auerbach's plexus and their axons extend in an anal direction before innervating mainly the circular muscle coat. In the stomach they are controlled by preganglionic cholinergic fibres of parasympathetic origin. They are involved in "receptive relaxation" of the stomach, "descending inhibition" in peristalsis and reflex relaxation of oesophageal and internal anal sphincters. The terminal varicosities of purinergic nerves are characterised by a predominance of "large opaque vesicles," which can be distinguished from the "large granular vesicles" found in small numbers in both adrenergic and cholinergic nerves. Stimulation of purinergic nerves with single pulses produces hyperpolarisations of up to 25 mV (inhibitory junction potentials) in smooth muscle cells. These potentials are unaffected by atropine, adrenergic neuron blocking agents or sympathetic denervation, but are abolished by tetrodotoxin. The "rebound contraction" which characteristically follows cessation of purinergic nerve stimulation is probably due to prostaglandin. Evidence that ATP is the transmitter released from purinergic nerves includes: (1) synthesis and storage of ATP in nerves; (2) release of ATP from the nerves when they are stimulated; (3) exogenously applied ATP mimicking the action of nerve-released transmitter, both producing a specific increase in K+ conductance; (4) the presence of Mg-activated ATPase, 5'-nucleotidase and adenosine deaminase, enzymes which inactivate ATP; (5) drugs (including quinidine, some 2-substituted imidazolines, 2-2'pyridylisatogen and dipyridamole) which produce similar blocking or potentiating effects on the response to exogenously applied ATP and nerve stimulation. Speculations are made about the evolution and development of the nervous system, including the possibility that purinergic nerves are a primitive nerve type.

Creep after loading in relaxed and contracted (KC1 or K2SO4 depolarized) smooth muscle (taenia coli of the guinea pig)

1. The time course of creep in the taenia coli of the guinea pig was recorded during 2000 sec (33 min 20 sec) in the relaxed (Ca2+-free bath solution with verapamil) and contracted (KCl or K2SO4 depolarized) states. 2. The variations in initial length before loading (l0), immediate elastic extension after loading (lE), and creep (N) were standardized with respoect to volume (cm/cm3) and compared among the different states. 3. Immediate elastic extension (lE) and particularly creep after 2000 sec (N2000) are minimal in the relaxed and maximal in the K2SO4-contracted state. The values in the KCl-contracted state are probably affected by intracellular swelling. Statistically, there is a significant difference between the overall length (l0 + lE + N2000) in the relaxed and in the K2SO4-contracted state when creep ends. 4. The hyperbolic relation of N to dN/dt representing the time course of creep mentioned in an earlier paper is reconfirmed for the relaxed as well as for the contracted state. 5. The parameters of the equation giving this relation are calculated from the experimental data. They characterize elastic properties and inner friction during creep. It is shown that the parameters of inner friction diminish more than those characterizing the elastic properties if the preparation is changed from the relaxed into the contracted state by K2SO4-depolarization. 6. In the discussion further evidence is given that not only changes in the diameter of the preparation but also changes of the intracellular elements must be responsible for the altered time course of creep during contraction.

Dual effects of calcium ions on pacemaker activity in guinea-pig taenia coli

The effects of various [Ca2+]0 on membrane potential (MP), action potential (AP) frequency, and isometric tension of isolated guinea-pig taenia coli were studied using intracellular recording techniques and simultaneous tension measurement. At 5.9 mM [K+]0 the order of potency of [Ca2+]0 this order is gradually reversed since high [Ca2+]0 becomes more potent in accelerating impulse discharges. At 2.5 mM [Ca2+]0 the line relating MP to log [K+]0 is not straight; its slope for a tenfold change of [K+]0 is 21.1 mV in the range between 5.9 and 17.7 mM [K+]0, and 51.5 mV between 32.45 and 59 mM [K+]0. In general, reducing [Ca2+]0 depolarizes the membrane whereas increasing [Ca2+]0 hyperpolarizes it. The Ca2+-induced changes of MP are reduced at high [K+]0. At 2.5 and 7.5 mM [Ca2+]0 the lines relating AP frequency and tension to the MP are nearly superimposed. In contrast, at 0.83 mM [Ca2+]0 the line is shifted to lower frequency and tension for all MP values studied. In conclusion, in the range of low [Ca2+]0 the system underlying pacemaker activity seems to be dependent on Ca2+ in two ways: 1. by an indirect negative action mediated by an increase of PK+ and by hyperpolarization of the membrane; 2. by a direct positive action which is not mediated by alterations of MP. In the range of normal and high [Ca2+]0 only potential-mediated Ca2+-effects determine AP frequency. The hypothesis is put forward that Ca2+ may carry the background inward current responsible for pacemaker activity.

The role of electrogenic sodium pumping in the response of smooth muscle to acetylcholine

1. Intracellular recording of membrane potential was made from the separated longitudinal muscle of the guinea-pig terminal ileum in physiological salt solution.2. When acetylcholine was washed from the tissue following a brief application the membrane repolarized and then hyperpolarized (;after-hyperpolarization') beyond the level existing before the application of acetylcholine.3. No after-hyperpolarization was observed following acetylcholine in potassium-free solution, in sodium-deficient (17 mM) solution, or in the presence of ouabain (1.7 x 10(-6)M). Repolarization under these conditions was delayed, especially after the membrane potential reached -20 to -30 mV, and was generally incomplete.4. The after-hyperpolarization was significantly (P < 0.01) greater when acetylcholine was applied in chloride-deficient (13 mM) solution.5. It was incidentally observed that the membrane potential in the presence of acetylcholine was more positive in potassium-free solution (significance P < 0.025), unchanged in chloride-deficient solution (P > 0.4), and much more negative in sodium-deficient (17 mM) solution (P << 0.001), confirming previous results using carbachol.6. When a 2 min application of 1.4 x 10(-6)M carbachol was made, the membrane potential 15-20 sec after beginning its application was not affected by ouabain (10(-5)M), but showed a significantly (P < 0.005) greater positive shift subsequently, so that the potential after 120 sec in carbachol was significantly (P < 0.025) more positive in the presence of ouabain. After 45 sec in 5.5 x 10(-5)M carbachol the membrane potential was also significantly (P < 0.005) more positive in the presence of ouabain (10(-5)M).7. Calculations based on hypotheses concerning the movements of sodium and potassium showed that the positive shift of the membrane potential in the presence of carbachol when sodium pumping was arrested, could be quantitatively explained by a decline in the sodium and potassium gradients across the membrane. It appeared that the electrogenic fraction of the sodium pumped was small in the presence of carbachol.8. It was concluded that the application of acetylcholine or carbachol (> 10(-6)M) to this smooth muscle disturbs the sodium and potassium gradients across the membrane. These disturbances are in a direction which stimulates electrogenic sodium pumping. Some limitation of depolarization results, and the increased electrogenic extrusion of sodium is responsible for the after-hyperpolarization which follows the application of acetylcholine.

Effects of electrogenic sodium pumping on the membrane potential of longitudinal smooth muscle from terminal ileum of guinea-pig

1. The membrane potential of the separated longitudinal muscle of the guinea-pig terminal ileum was recorded intracellularly with glass micro-electrodes.2. In tissues kept at room temperature and then brought to 35 degrees C for 15-30 min or about 1 hr, the fall in membrane potential upon changing to potassium-free solution was 21.4 +/- 3.5 mV and 13.4 +/- 1.8 mV respectively. Ouabain (1.7 x 10(-6)M) produced a fall in membrane potential of 8.1 +/- 1.1 mV. Returning potassium to potassium-free solution, or changing from ouabain-containing to ouabain-free solution, resulted in an increase in membrane potential which was greater than the initial fall.3. Readmitting potassium to potassium-free solution produced an increase in membrane potential which began within 10 sec and reached a maximum within 15-30 sec. This response was reduced, abolished, or converted to a depolarization by ouabain. In chloride-deficient (13 mM) solution in which membrane resistance was increased, the response to readmitting potassium was increased 2(1/2)-fold so that the membrane potential sometimes exceeded -100 mV, which was probably more negative than E(K). On the basis of these results it was assumed that the response to readmitting potassium was due to the electrogenic activity of the sodium pump.4. The response to briefly readmitting a fixed concentration of potassium increased during the first 30 min in potassium-free solution. This increase was not due to an increase in membrane resistance as this fell with time in potassium-free solution. It was suggested that the increase in the response resulted from the progressive rise in internal sodium concentration which is known to occur in smooth muscle in potassium-free solution.5. Increasing the concentration of potassium over the range approximately 0.1-20 mM, increased the size of the electrogenic potential observed upon readmitting potassium to potassium-free solution. There was a fall in membrane resistance upon readmitting potassium (0.6, 5.9, or 20 mM) which was greater the larger the concentration of potassium. When allowance was made for the fall in membrane resistance, the dependency of the electrogenic response upon the concentration of potassium over the range 0.6-20 mM was much increased.6. The results indicate that the rate of electrogenic sodium pumping in this tissue is increased by increasing the external potassium concentration, and probably by increasing the internal sodium concentration. It was suggested that a rise in the latter could sensitize the pump to an increase in the former.

Membrane potential of smooth muscle cells in K-free solution

1. The changes of the ion content, the membrane potential and of the membrane permeability of taenia coli cells have been studied during exposure to K-free solutions. The relative value of the total membrane conductance was determined by measuring the electrotonic potential during constant current pulses with an intracellular electrode. The P(K) values were calculated from (42)K-efflux in K-free solutions.2. In solutions containing penetrating anions the cells initially depolarize. Thereafter they hyperpolarize to about - 85 mV and again depolarize after 90 min to - 5 mV. These potential changes are much smaller if large anions are used as chloride substitutes. Moreover, the final depolarization is only reached after 4-5 hr. This hyperpolarization is not inhibited by 10(-5)M ouabain.3. These potential changes are accompanied by a progressive exchange of intracellular K by Na. In solutions containing chloride or nitrate the relative value of the total membrane conductance increases to a maximal value, corresponding to the peak value of the calculated P(K). Such changes of the membrane conductance and of P(K) do not occur in K-free solutions containing large anions.4. It is proposed that the initial depolarization is probably caused by an inhibition of an electrogenic Na pump. In chloride or nitrate solution the hyperpolarization is due to an increase of the [K](i)/[K](o) ratio and to an increase of the K permeability. In the presence of large anions the hyperpolarization remains small because this increase of P(K) does not occur.

The mechanism of acetylcholine release from parasympathetic nerves

1. The output of acetylcholine from the plexus of the guinea-pig ileum longitudinal strip has been used to study the mechanism of acetylcholine release. From the effects of hexamethonium and tetrodotoxin, it was inferred that 60% of the normal resting output is due to propagated activity in the plexus, and 40% to spontaneous release. Tetrodotoxin virtually abolishes the increase in output in response to electrical stimulation.2. Resting acetylcholine output is increased when the bathing medium is changed in the following ways:(a) sodium replacement by sucrose, trometamol or lithium;(b) addition of ouabain or p-hydroxymercuribenzoate (PHMB), or withdrawal of potassium;(c) the combination of PHMB and partial sodium replacement;(d) addition of potassium; this increase in output becomes greater in the absence of sodium.3. The resting output is virtually abolished by calcium withdrawal, and is restored by barium substitution for calcium. It is also reduced by raising the magnesium concentration.4. The enhanced resting output in response to sodium withdrawal also occurs in the absence of calcium.5. Cooling to 5 degrees C greatly reduces both the resting output and the output in response to raised potassium concentration or to electrical stimulation.6. The increase in resting output due to potassium excess is slight up to 25 mM [K(+)](o), but increases thereafter with about the fourth power of the potassium concentration; it is resistant to tetrodotoxin.7. Synthesis of acetylcholine by the longitudinal strip is increased when output is enhanced by electrical stimulation, by potassium excess or by addition of barium, so that the acetylcholine content of the strip is maintained approximately normal. Synthesis is reduced, in relation to output, by potassium lack or by treatment with ouabain, and is virtually abolished by sodium withdrawal.8. The theory is discussed that acetylcholine release depends on inhibition of the activity of a (Na(+) + K(+) + Mg(2+))-activated ATPase at the axonal membrane.

Effects of removing the external potassium on the smooth muscle of guinea-pig taenia coli

1. Removal of the external K ions depolarized the membrane of the guinea-pig taenia coli by about 5 mV, and increased the frequency of the spontaneous spike activity. After about 20 min in K-free solution, the frequency gradually decreased and spontaneous activity stopped after about 60 min.2. Readmission of K produced a transient hyperpolarization by 10-15 mV for 10-20 min, and lowered the excitability. The membrane resistance decreased during the hyperpolarization. When the external K concentration was returned to normal before the spontaneous activity had disappeared in K-free solution, there was a complete suppression of the spontaneous electrical and mechanical activities, accompanied by hyperpolarization of the membrane. The duration of this suppression of electrical and mechanical activities depended on the exposure time to K-free solution, the recovery being delayed by longer exposure.3. The effects of removing external K were essentially the same in the presence of tetrodotoxin (10(-7) g/ml.), of atropine (10(-6) g/ml.), and also in low Cl (2-7 mM).4. The effects of ouabain (10(-6) g/ml.) were similar to those of zero K but appeared more slowly. After treatment with ouabain, the membrane was hyperpolarized by removing the external K and there was no hyperpolarization nor suppression of the mechanical activity on readmission of external K.5. Rb substitution for K produced little effect and addition of 5.9 mM-Rb after K-removal produced a typical suppression of activity as the readmission of K. When K was replaced with an equimolar concentration of Cs, the effects were similar to those produced by K-free solution.6. The pattern of the response to a change in the external K concentration was different below 23 degrees C. The spontaneous activity was suppressed by removal of K, and increased by returning the external K.7. It is concluded that, in the taenia, the membrane potential can be modified by an electrogenic pump, the activity of which is inhibited by removing K, by ouabain, and by lowering the temperature to about 23 degrees C. During the recovery from the exposure to K-free solution or to ouabain, the pump activity is potentiated, causing hyperpolarization of the membrane by 10-20 mV. Rb can, Cs cannot, replace K in activating the pump at the same concentration as K.

The relative contribution of K and Cl to the total increase of membrane conductance produced by adrenaline on the smooth muscle of guinea-pig Taenia coli

1. The relative contribution of K and Cl to the total increase of membrane conductance produced by adrenaline on the smooth muscle of guinea-pig taenia coli has been investigated. Constant current pulses were applied with a pair of large external electrodes, the voltage change was recorded with an intracellular micro-electrode, and the change in the size of the electrotonic potential by adrenaline was measured.2. Adrenaline hyperpolarized the membrane by about 8 mV with a range from 5 to 15 mV and it reduced both the size and the time course of the electrotonic potential. On the average the size was reduced to 65 +/- 1.8% (n = 75) of the normal size, from which a decrease of the membrane resistance to 45 +/- 2.2% was calculated, or 48% allowing for the spatial decay.3. The magnitude of the reduction of the electrotonic potential caused by adrenaline was decreased as the concentration of K or Cl ions in the medium was reduced by substituting NaCl or sucrose for KCl or an impermeant anion, benzene sulphonate, for Cl ions. The adrenaline effect remained unaltered when the NaCl was replaced with Tris-Cl.4. It is therefore concluded that adrenaline opens the ion pathways mainly for K and Cl.5. On the basis of the changes in potential and in conductance caused by adrenaline, the equilibrium potential for its action was calculated as - 75 mV.6. The ratio of the Cl component to the K component of the additional conductance increase was obtained as 0.36 using the calculated equilibrium potential and the Nernst potentials for K (-91 mV) and Cl (-31 mV).

The action potential in the smooth muscle of the guinea pig taenia coli and ureter studied by the double sucrose-gap method

The configuration of the electrotonic potential and the action potential observed by the double sucrose-gap method was similar to that observed with a microelectrode inserted into a cell in the center pool between the gaps. In the taenia and the ureter, the evoked spike was larger in low Na or in Na-free (sucrose substitute) solution than in normal solution. However, the plateau component in the ureter was suppressed in the absence of Na. In Ca-free solution containing Mg (3-5 mM) and Na (137 mM), the membrane potential and membrane resistance were normal, but no spike could be elicited in both the taenia and ureter. Replacement of Ca with Sr did not affect the spike in the taenia, nor the spike component of the ureter but prolonged the plateau component. The prolonged plateau disappeared on removal of Na, while repetitive spikes could still be evoked. It was concluded that the spike activity in the taenia and in the ureter of the guinea pig is due to Ca entry, that the plateau component in the ureter is due to an increase in the Na conductance of the membrane, and that both mechanisms, for the spike and for the plateau, are separately controlled by Ca bound in the membrane.

Calculation of the membrane potential in smooth muscle cells of the guinea-pig's taenia coli by the Goldman equation

1. The intracellular K(+), Cl(-) and Na(+) concentrations in the taenia coli cells of the guinea-pig have been estimated from the total ion content or the extrapolated intracellular tracer content, the sorbitol space and the dry wt./wet wt. ratio.2. The exchange of K(+), Cl(-) and Na(+) was studied by following the uptake and the efflux of these ions with radioactive isotopes. The following efflux values have been calculated: m(K), 4 p-mole.cm(-2).sec(-1); m(Cl), 8.4 p-mole.cm(-2).sec(-1) and m(Na), 7.2 p-mole.cm(-2).sec(-1). These flux values agree well with the influx values, obtained under the same experimental conditions.3. The slowness of diffusion in the extracellular space reduces the Na flux by about 2.5% and the K flux by about 30%. A correction factor of 1.3 has to be introduced to obtain the true K flux.4. The values for the permeability constants calculated by the constant field assumptions are for P(K), 11 x 10(-8) cm/sec; P(Cl), 6.7 x 10(-8) cm/sec and for P(Na), 1.8 x 10(-8) cm/sec. The introduction of these values and of the ion concentrations in the Goldman equation gives a resting potential of -37 mV.5. One of the possible explanations for the discrepancy between the measured resting potential and the calculated one, is that the resting potential of these smooth muscle cells is partly a diffusion potential and partly due to the operation of an electrogenic Na pump.

Effect of calcium, barium and manganese on the action of adrenaline in the smooth muscle of the guinea-pig taenia coli

(1) The effect of calcium, barium and manganese on membrane conductance and spike activity of intestinal smooth muscle, and their influence on the action of adrenaline, was studied. (2) The hyperpolarization and stabilization of the membrane by exposure to excess Ca was associated with a decrease of membrane resistance. Reduction of the external Ca increased membrane resistance. However, with further reduction to zero Ca the membrane resistance became very low; the membrane was depolarized and spike activity was abolished. (3) The addition of Ra (0*25 to 1*0 mM) to Ca-free medium caused repolarization of the membrane with an increase in membrane resistance and recovery of spike activity. The spike amplitude was increased by about 25 mV per tenfold increase in the external Ba concentration. (4) Higher Ba concentrations (1*0 to 2*5 mM) prolonged the spike duration resulting in plateau formation and, finally, in a state of sustained depolarization. This could be term inated either by inward current application or by the addition of Ca which restored spikes. As the external Ca concentration was increased the Ba-effect became less, indicating a competition between Ca and Ba. (5) In the presence of Ca, 0*5 mM Mn abolished spontaneous spike discharge and 2*5 mM Mn also abolished evoked spikes. In Ca-free solution, addition of Mn caused repolarization of the membrane with recovery of membrane resistance, but spike activity remained absent. (6) The action of adrenaline was potentiated by excess Ca and decreased by lowering the external Ca concentration. (7) Adrenaline had no effect when Ca had been substitued by Ba. Traces of Ca restored tho action of adrenaline. (8) Mn abolished the action of adrenaline which was not restored by adding Ca. (9) The observations support the view that Ca is im portant for controlling membrane conductance, that the spike in taenia coli is due to Ca-entry and that Ca is essential for the action of adrenaline. Mn can replace Ca for membrane stabilization, it cannot replace Ca for the spike nor for the adrenaline action. Ba can, to some extent, replace Ca for membrane stabilization and Ba is effective in producing the spike, but Ba cannot replace Ca for the action of adrenaline.

The effect of sodium and calcium on the action potential of the smooth muscle of the guinea-pig taenia coli

1. Spontaneous spike activity and action potentials evoked by external field stimulation were recorded, intracellularly and with the double sucrose gap method, from the smooth muscle of guinea-pig taenia coli.2. Replacement of external NaCl with sucrose (leaving 10 mM-Na in the buffer) caused hyperpolarization and stopped spontaneous activity within 10 min. Spikes could, however, be evoked for 2-3 hr. The amplitude, the overshoot and rate of rise of the spike were increased.3. In 10 mM-[Na](o) the intracellular Na concentration was reduced from 35 to 24 mM, shifting the Na-equilibrium potential from +34 to -22 mV.4. Excess Ca (12.5 mM) caused hyperpolarization and increased membrane conductance. The amplitude and the rate of rise of the spike were increased, the threshold was raised and the latency of the spike evoked by threshold stimulation became shorter.5. The effect of reducing the external Ca concentration depended on the Na concentration present, being greater with higher external [Na](o). When the membrane was depolarized and spikes deteriorated in low Ca (0.2-0.5 mM) reduction of Na to 10 mM caused repolarization and recovery of the action potential.6. Mn (0.5-1.0 mM) blocked spontaneous spike discharge after 20 min. Higher concentrations (more than 2.0 mM) were required to block the evoked action potential.7. The results indicate that the smooth muscle spike in taenia is due to Ca-entry and that Na influences spike activity indirectly by competing with Ca in controlling the membrane potential.

Influence of the ionic environment on spontaneous electrical and mechanical activity of the rat portal vein

The effects of variations in external K + and Ca 2+ concentrations on electrical and mechanical activity of the isolated rat portal vein were studied using a sucrose-gap technique. Spontaneous activity in normal Krebs solution consists of bursts of action potentials accompanied by phasic contractions. Moderate reduction in [K+] o results in slight hyperpolarization with decreased discharge frequency. Greater reduction or total removal of extracellular K causes a temporary increase in burst frequency. Graded increases in [K+] o from 6 up to 128 mM cause depolarization with an increase in discharge frequency and in contraction amplitude until [K+] o reaches a level of 6 to 7 times normal when spike potentials are abolished and a contracture develops. The relationship between membrance potential and log [K + ] o indicates a relatively low permeability to K + . Increasing [Ca 2+ ] o causes slight hyperpolarization, decreased burst frequency, increased burst duration and increased contraction amplitude. Reducing [Ca 2+ ] o causes increased frequency of discharge and shortening of bursts, until single spikes appear, each preceded by a prepotential; thus the activity resembles that in cardiac pacemaker cells. In Ca 2+ -free solution, electrical and mechanical activity ceases within 10 to 15 min. Calcium is essential for activation of the contractile mechanism and for maintenance of spontaneous spike discharge.

The calcium content of the smooth muscle of the guinea-pig taenia coli

1. The in vitro calcium content of the smooth muscle of the guinea-pig taenia coli was 3.0 m-mole Ca/kg wet wt. when phosphate was omitted from the bathing medium, and was almost independent of pH changes in the range 6.7-7.6.2. The calcium content was not changed when 1 mM phosphate was included in the medium, if the pH was 6.7 or 7.0. However, when the pH was 7.6, the calcium content increased by 1.5 m-mole Ca/kg wet wt. in the presence of phosphate.3. The calcium content rose by 1.1 m-mole Ca/kg wet wt. when NaCl in the bathing medium was replaced by isotonic sucrose, and rose by 0.7 m-mole Ca/kg wet wt. when MgCl(2) in the bathing medium was replaced. These increases may reflect a competition between Ca(2+) and other cations for fixed negative sites in the tissue.4. The initial rapid phase of (42)K exchange corresponded to an ;extra-cellular (42)K-space' of 470 ml./kg fresh wt. in normal solution, rising to 560 ml./kg. fresh wt. in low-sodium solution and to 760 ml./kg fresh wt. in calcium-free low-sodium solution. In this last medium the extra-cellular [(14)C]sorbitol space was only 390 ml./kg fresh wt., so that there was a large excess of rapidly-exchanging potassium which may have been competing at fixed negative sites.

Spike propagation in the smooth muscle of the guinea-pig taenia coli

1. Spike activity was produced by external field stimulation of the guinea-pig taenia coli. Spikes were evoked by depolarization of the muscle membrane. Though the activity was usually also observed during hyperpolarization, this was shown to be conducted activity from the depolarized region of the tissue. Spike conduction was blocked when hyperpolarization exceeded 10 mV.2. The shape of the conducted spike was influenced by membrane polarization. Sometimes a notch appeared on the spike and sometimes the spike was split into two by hyperpolarization. This is probably due to the fact that functional bundles form a network and that the branches between bundles are more susceptible to the membrane polarization.3. There was a critical spike amplitude for normal propagation. Therefore, different spike amplitudes were observed near the stimulating electrode, but only spikes of nearly full size amplitude were recorded far from the stimulating electrode, i.e. at a distance of more than 5 mm.4. When repetitive stimulation was applied, the spike amplitude decreased with increasing frequency of stimulation. No steady level was reached, however, but the spike amplitude fluctuated at about 0.3 c/s.