Cholinergic agonists suppress a potassium current in freshly dissociated smooth muscle cells of the toad

Properties of the cromakalim-induced potassium conductance in smooth muscle cells isolated from the rabbit portal vein

1. Single smooth muscle cells were isolated freshly from the rabbit portal vein and membrane currents were recorded by the whole-cell or excised patch configurations of the patch-clamp technique at room temperature. 2. Cromakalim (Ckm, 10 microM) induced a potassium current (ICkm) that showed no pronounced voltage-dependence and had low current noise. 3. This current, ICkm, was inhibited by (in order of potency): phencyclidine greater than quinidine greater than 4-aminopyridine greater than tetraethylammonium ions (TEA). These drugs inhibited the delayed rectifier current, IdK, which is activated by depolarization of the cell, with the same order of potency. 4. Large conductance calcium-activated potassium channels (LKCa) in isolated membrane patches were blocked by (in order of potency) quinidine greater than TEA approximately phencyclidine. 4-Aminopyridine was ineffective. A similar order of potency was found for block of spontaneous transient outward currents thought to represent bursts of openings of LKCa channels. 5. The low current noise of ICkm at positive potentials, and its susceptibility to inhibitors indicated that it was not carried by LKCa channels, and that it may be carried by channels which underlie IdK. It was observed that when ICkm was activated, IdK was reduced. However, in two experiments, ICkm was much more susceptible to glibenclamide than IdK; possible reasons for this are discussed.

Arachidonic acid and other fatty acids directly activate potassium channels in smooth muscle cells

Arachidonic acid, as well as fatty acids that are not substrates for cyclooxygenase and lipoxygenase enzymes, activated a specific type of potassium channel in freshly dissociated smooth muscle cells. Activation occurred in excised membrane patches in the absence of calcium and all nucleotides. Therefore signal transduction pathways that require such soluble factors, including the NADPH-dependent cytochrome P450 pathway, do not mediate the response. Thus, fatty acids directly activate potassium channels and so may constitute a class of signal molecules that regulate ion channels.

Muscarinic receptor modulation of glucose-induced electrical activity in mouse pancreatic B-cells

Acetylcholine (1-10 microM) depolarized the membrane and stimulated glucose-induced bursts of electrical activity in mouse pancreatic B-cells. The acetylcholine effects were mimicked by muscarine while nicotine had no effect on membrane potential. Pirenzepine, an antagonist of the classical M1-type muscarinic receptors, but not gallamine (1-100 microM), an antagonist of the classical M2-type receptors, antagonized the acetylcholine action on glucose-induced electrical activity (IC50 = 0.25 microM). Bethanechol, an agonist of the classical M2-type muscarinic receptors, was approximately 100 times less effective than acetylcholine in stimulating the electrical activity. In addition, acetylcholine (1 microM) induced a marked increase (25%) in input resistance to the B-cell membrane. The results indicate that acetylcholine exerted its effects on the B-cell membrane by inhibiting K+ conductance via activation of a muscarinic receptor subtype distinct from the classical M2-type receptor.

Regulation of calcium concentration in voltage-clamped smooth muscle cells

The regulation of intracellular calcium concentration in single smooth muscle cells was investigated by simultaneously monitoring electrical events at the surface membrane and calcium concentration in the cytosol. Cytosolic calcium concentration rose rapidly during an action potential or during a voltage-clamp pulse that elicited calcium current; a train of voltage-clamp pulses caused further increases in the calcium concentration up to a limit of approximately 1 microM. The decline of the calcium concentration back to resting levels occurred at rates that varied with the calcium concentration in an apparently saturable manner. Moreover, the rate of decline at any given calcium concentration was enhanced after a higher, more prolonged increase of calcium. The process responsible for this enhancement persisted for many seconds after the calcium concentration returned to resting levels. Thus, the magnitude and duration of a calcium transient appear to regulate the subsequent calcium removal.

Membrane ionic mechanisms activated by noradrenaline in cells isolated from the rabbit portal vein

1. Membrane currents were recorded in cells freshly dispersed from the rabbit portal vein with patch pipette techniques in the whole-cell configuration of recording. In potassium-containing solutions at a holding potential of -50 mV noradrenaline usually evoked an inward current and enhanced greatly the outward current evoked by depolarizing voltage steps. 2. The ionic mechanism of the inward current was investigated in potassium-free solutions in which the inward current elicited by noradrenaline was produced by an increase in membrane conductance. 3. In the first series of experiments NaCl was the main salt in the patch pipette solution (representing the intracellular milieu) and the ionic composition of the bathing solution was altered. In these conditions the reversal potential of the noradrenaline-induced current was close to the chloride equilibrium potential (ECl). 4. When sodium glutamate was the major salt in the pipette solution the reversal potential of the noradrenaline-evoked current was influenced by the cation rather than the anion gradient. 5. With 89 mM-BaCl2 in the external solution (and sodium glutamate in the pipette) noradrenaline produced a membrane current with a reversal potential which was much more positive than ECl or ENa. These results indicate that noradrenaline opens a chloride-selective channel and a cation channel which is permeable to monovalent and divalent cations. 6. Bath application of 10 mM-caffeine evoked a membrane current with a reversal potential close to ECl with either NaCl or sodium glutamate in the pipette. This is interpreted to mean that the increase in membrane chloride conductance can occur as a consequence of a rise in intracellular calcium concentration. It is less evident that the cation channel is calcium activated.

Beta-adrenergic actions on membrane electrical properties of dissociated smooth muscle cells

Studies were carried out to determine the effects of the beta-adrenergic agent, isoproterenol (ISO), on membrane electrical properties in single smooth muscle cells enzymatically dispersed from toad stomach. In cells bathed in buffer of physiological composition, the average resting potential was -56.4 +/- 1.4 mV (mean +/- SE, n = 35). The dominant effect of exposure to ISO was hyperpolarization. The hyperpolarization was apparent in all cells studied and averaged 11.6 +/- 1.2 mV (n = 27). In the majority of the cells, hyperpolarization was accompanied by a decreased input resistance (Rin). Often the change in resistance appeared to lag behind the change in membrane potential. The lack of coincident changes in membrane potential and resistance may reflect a superposition of the outward rectification properties of the membrane on beta-adrenergic-induced increases in ionic conductance. In about half of the cells, an initial small depolarization (3.1 +/- 0.3 mV, n = 14) was accompanied by a small but distinct increase in Rin (12 +/- 2.5%). When membrane potential was made more negative than the estimated equilibrium potential for K+ (EK) by injection of current, ISO also produced biphasic effects, an initial hyperpolarization which reversed to a sustained depolarization to a value (-90 mV) near the estimated EK. The hyperpolarization by ISO could be diminished in a time-dependent manner by previous exposure to ouabain. The inhibition by ouabain, however, appeared to be a fortuitous result of glycoside-induced positive shifts in EK. These observations indicate that the dominant electrophysiological effect of beta-adrenergic stimuli is to hyperpolarize the cell membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of known K+-channel blockers on the electrical activity of bovine lymphatic smooth muscle

The effects of known K+-channel blockers on the electrical properties of bovine lymphatic smooth muscle were investigated using the double sucrose-gap technique. Constant current anodal pulses elicited hyperpolarizing electrotonic potentials (EP's) which were characterised by a "sag" in the potential record. Current/voltage relationship (I/V), which were examined by measuring EP amplitude at the end of 5 s anodal pulses (less than 30 microA), showed an apparent increase in conductance with increasing hyperpolarization. In the presence of caesium (10 mM), 4-aminopyridine (10 mM) or in the absence of external K+ the sag in the EP was lost and the inward rectification characteristic of the control I/V relationship was abolished. Barium (2.5 mM) also abolished in sag in the EP although TEA (10 mM) had no effect on either EP shape or I/V relationship. Thus it would appear that lymphatic smooth muscle shows inward rectification which is slowly activating and is blocked by some of the known K+-channel blockers or by the removal of external K+.

Antagonistic adrenergic-muscarinic regulation of M current in smooth muscle cells

The beta-adrenergic agonist isoproterenol and analogs of adenosine 3',5'-monophosphate (cAMP) induced a potassium current, M current, in freshly dissociated gastric smooth muscle cells. Muscarinic agonists suppress this current, apparently by acting at a locus downstream from regulation of cAMP levels by adenylate cyclase and phosphodiesterase. Thus, M current can be induced by an agent and regulated in antagonistic fashion by beta-adrenergic and muscarinic systems.

Membrane mechanism associated with muscarinic receptor activation in single cells freshly dispersed from the rat anococcygeus muscle

1 The mechanism of action of carbachol was studied on freshly dispersed cells of the rat anococcygeus using microelectrodes and patch pipettes. 2 Micro-ionophoretic application of carbachol evoked reproducible depolarizations which were reduced or blocked by atropine (10(-7)-10(-6) M). The time courses of the responses to noradrenaline and carbachol were similar. 3 The reversal potential of the carbachol-induced response was -3.8 mV and similar to the value (-6.2 mV) found for noradrenaline. 4 During the response to carbachol the membrane conductance was increased. At depolarized membrane potentials carbachol evoked biphasic membrane responses suggesting an increase in two separate ionic conductances. 5 With patch pipettes in the whole-cell configuration under voltage-clamp, carbachol produced an inward current at a holding potential of -50 mV. The inward current was associated with an increase in membrane conductance with an equilibrium potential of about 0 mV. 6 It is suggested that muscarinic receptors and adrenoceptors in the rat anococcygeus may activate similar membrane conductances. The most prominent mechanism is an increase in chloride ion conductance.

Isolated muscle cells as a physiological model

Summary of a symposium presented by the American Physiological Society (Cell and General Physiology Section and Muscle Group) at the 70th Annual Meeting of the Federation of American Societies for Experimental Biology, St. Louis, Missouri, April 15, 1986, chaired by M. Lieberman and F. Fay. This symposium reflects a growing interest in seeking new technologies to study the basic physiological and biophysical properties of cardiac, smooth, and skeletal muscle cells. Recognizing that technical and analytical problems associated with multicellular preparations limit the physiological significance of many experiments, investigators have increasingly focused on efforts to isolate single, functional embryonic, and adult muscle cells. Progress in obtaining physiologically relevant preparations has been both rapid and significant even though problems regarding cell purification and viability are not fully resolved. The symposium draws attention to a broad, though incomplete, range of studies using isolated or cultured muscle cells. Based on the following reports, investigators should be convinced that a variety of experiments can be designed with preparations of isolated cells and those in tissue culture to resolve questions about fundamental physiological properties of muscle cells.

Acetylcholine activates single sodium channels in smooth muscle cells

Using the whole-cell clamp technique with a patch electrode, single channel activities were recorded in dispersed single smooth muscle cells of the guinea-pig ileum in response to application of greater than 1 microM acetylcholine (ACh) or carbachol (CCh). Under physiological conditions (bath:Krebs, pipette:High K), these channels have a single channel conductance of 20-25 pS at the membrane potentials ranging between -100 and -40 mV and are activated in the membrane potential dependent manner. The amplitude of the channel current showed a strong dependence on the extracellular Na concentration but the reversal potential obtained by the extrapolation of the I-V relationship was not consistent with the equilibrium potential of Na ion. An approximate estimation of permeability ratio based on the independent principle described by Hodgkin et al. gave a value of Na:K = 1.0:0.3-0.4 to this channel. From features of the macroscopic and single channel currents, it is concluded that these muscarinic ACh(CCh)-activated channels mainly pass Na ion and play a major part in the membrane depolarization produced by ACh or CCh in mammalian intestinal smooth muscles.

Acetylcholine increases voltage-activated Ca2+ current in freshly dissociated smooth muscle cells

The regulation of voltage-activated Ca2+ current by acetylcholine was studied in single freshly dissociated smooth muscle cells from the stomach of the toad Bufo marinus by using the tight-seal whole-cell recording technique. Ca2+ currents were elicited by positive-going command pulses from a holding level near -80 mV in the presence of internal Cs+ to block outward K+ currents. Ca2+ current was greatest in magnitude at command potentials near 10 mV. At such command potentials, acetylcholine increased the magnitude of the inward current and slowed its decay. The effects of acetylcholine were seen in the absence of external Na+ or with low Cl- (aspartate replacement) in the bathing solution and could be mimicked by muscarine. The peak of the current-voltage relationship for the Ca2+ current was not discernibly shifted along the voltage axis by acetylcholine. These results demonstrate that activation of muscarinic receptors not only suppresses a K+ current (M-current), as we have previously demonstrated [Sims, S. M., Singer, J. J. & Walsh, J. V., Jr. (1985) J. Physiol. (London) 367, 503-529], but also increases the magnitude and slows the decay of Ca2+ current.

Identification and characterization of major ionic currents in isolated smooth muscle cells using the voltage-clamp technique

Voltage-clamp experiments were carried out on freshly dissociated single vertebrate smooth muscle cells from the stomach muscularis of Bufo marinus. Conventional two-microelectrode methodology was used, thus avoiding rapid dialysis of the cytosol. Four major phases of current were identified upon voltage jumps from negative holding levels to more positive levels. The first phase of current was an initial, inward current. This current was blocked by external Mn2+ and was of the correct magnitude to account for the rising phase of the Ca2+-dependent, TTX-independent action potentials found in these cells. Following this initial, inward Ca2+ current, a large outward current was observed which reached its peak over a period of hundreds of milliseconds and then decayed over a period of seconds to a steady-state level. The peak outward current and the steady-state outward current constitute the second and third major currents. The peak outward current was the largest current observed, with a magnitude as large as tens of nanoamps whereas the inward current was at most about one nanoamp. The peak outward current was reduced more than tenfold in the presence of external TEA. It was also decreased or abolished when the preceding inward current was diminished or eliminated by using external Mn2+ or less negative holding potentials. In this way the peak outward current was identified as a Ca2+-activated K+ current whose slow decay was hypothesized to result from removal of internal Ca ions by cellular mechanisms following the initial rise in [Ca2+]i resulting from the inward current. A fourth major current was an early transient outward current observed most clearly upon voltage jumps to more positive potentials when the inward current was eliminated by using less negative holding potentials or external Mn2+. A classical steady-state inactivation relationship as a function of membrane potential was constructed for the inward current. A substantial portion of this inactivation curve lies at potentials negative to the apparent threshold for activation of inward current, suggesting a true voltage-dependent inactivation. Although additional Ca2+-dependent inactivation could not be ruled out, neither could evidence for it be found.

Inward rectification in freshly isolated single smooth muscle cells of the rabbit jejunum

1. Single smooth muscle cells were obtained by collagenase digestion of longitudinal muscle of rabbit jejunum. Membrane potential or current under voltage clamp was recorded by patch-clamp technique in the whole-cell recording mode at 22-25 degrees C. 2. At a membrane potential of -50 mV small hyperpolarizing current pulses produced electrotonic potentials which asymptotically approached a steady-state size and did not 'sag'. Stronger hyperpolarization resulted in a 'sag' of the electrotonic potential. Under voltage clamp an inward current, i, was activated in the range -60 to -110 mV. 3. This current had an equilibrium potential of -24.5 +/- 3.5 mV which was shifted negatively by reducing the sodium concentration, and positively by raising the potassium concentration of the bathing solution. 4. This current was blocked by caesium (1 mM) but less affected by barium ions up to 10 mM. 5. The time course of i upon stepping into its activation range was accelerated by increasing negativity and following an initial short delay could be described by a single exponential with a time constant in the range 60 s(-60 mV) to 1 s(-130 mV). 6. It is concluded that these jejunal cells possess a current to which sodium and potassium ions make a contribution which is responsible for the inward rectification they show upon hyperpolarization. This current is activated in a range which would allow it to contribute to the slow potential changes shown by longitudinal jejunal muscle.

Substance P and acetylcholine both suppress the same K+ current in dissociated smooth muscle cells

The effect of substance P on freshly dissociated gastric smooth muscle cells was examined electrophysiologically. Substance P caused depolarization, associated with a membrane conductance decrease, which led to the generation of action potentials and contraction. When the membrane potential was held constant under voltage clamp, substance P induced a net inward current, also associated with a conductance decrease. The net inward current resulted from suppression of an outward K+ current, one which resembled the acetylcholine-sensitive M-current in these cells. When substance P maximally suppressed this outward K+ current, acetylcholine (ACh) had no additional effect. Conversely, when ACh fully suppressed the M-current, substance P was without additional effect. These results indicate that substance P suppresses the same outward K+ current affected by ACh. Suppression of M-current by substance P was observed in approximately half (44 of 85) of the cells studied in these experiments. In those cells that did not respond to substance P, ACh was nevertheless capable of suppressing the M-current. Thus both substance P and cholinergic agonists appear to exert their excitatory effects on smooth muscle cells by inhibiting a common K+ current.

Multiple neuropeptides exert a direct effect on the same isolated single smooth muscle cell

The contractile effect of various neuropeptides was examined by pressure ejecting these agents from a pipette onto single smooth muscle cells freshly dissociated from the stomach of Bufo marinus. Substance P, cholecystokinin-octapeptide, and bombesin caused contraction, whereas vasoactive intestinal peptide, secretin, and dopamine inhibited acetylcholine-induced contractions. Acetylcholine and the three peptides which produced contraction were found in some instances to act on the same cell, suggesting that receptors for these agents exist on one and the same cell.

Large-conductance Ca2+-activated K+ channels in freshly dissociated smooth muscle cells

Freshly dissociated cells from the stomach muscularis of the toad Bufo marinus have been employed to carry out a systematic set of electrophysiological studies on the membrane properties of smooth muscle. The existence of Ca2+-activated K+ channels became apparent during the first studies under current clamp. In subsequent studies under voltage clamp, a Ca2+-activated. TEA-sensitive outward current was evident, and it was more than an order of magnitude larger than any other current observed in the cells. The channel responsible, at least in part, for this large outward current has been identified on the basis of single-channel records, and some of its main characteristics have been studied. It is similar in many respects to the large-conductance, Ca2+-activated K+ channel seen in other preparations. This channel has now been found in a considerable diversity of smooth muscle types.