https://www.cell.com/trends/pharmacological-sciences/pdf/0165-6147(88)90238-6.pdf

The pharmacology of potassium channels and their therapeutic potential

Cromakalim, nicorandil and pinacidil: novel drugs which open potassium channels in smooth muscle.

Interstitial cells of Cajal as targets for pharmacological intervention in gastrointestinal motor disorders

BRL34915 (0.1-10 microM) suppressed the spontaneous tone of guinea-pig isolated trachealis in a concentration-dependent manner. BRL34915 was not antagonized by propranolol (1 microM). In trachea where spontaneous tone was suppressed by indomethacin (2.8 microM) but subsequently restored to the same level with acetylcholine or histamine, the relaxant potency of BRL34915 was reduced. In Krebs solution containing K+ (120 mM), isolated trachealis muscle developed near-maximal tension. The relaxant effects of BRL34915 were virtually abolished in this medium. Concentration-effect curves for KCl, acetylcholine and histamine were constructed in tissues treated with indomethacin (2.8 microM). BRL34915 (10 microM) depressed the foot of the concentration-effect curve for KCl and caused minor rightward shifts in the concentration-effect curves of acetylcholine and histamine. Four K+-channel inhibitors were tested. Apamin (0.1 microM) did not modify the action of BRL34915. Tetraethylammonium (8 mM) had little effect but procaine (5 mM) and 4-aminopyridine (5 mM) each significantly inhibited the relaxant action of BRL34915. Intracellular electrophysiological recording showed that BRL34915 (0.1 microM) caused very minor relaxation and little, if any, electrical change. Higher concentrations (1-10 microM) evoked relaxation, suppression of spontaneous electrical slow waves and marked hyperpolarization of the trachealis cells. In the presence of TEA (8 mM) or procaine (5 mM) the hyperpolarization induced by BRL34915 was significantly reduced. In trachealis skinned of its plasma membranes, tension development induced by Ca2+ (20 microM) was unaffected either by BRL34915 (10 microM) or by nicorandil (1 mM). In studies of the efflux of 86Rb+ from muscle-rich strips of trachea, BRL34915 (1 and 10 microM) increased the efflux rate constant. It is concluded that BRL34915 evokes relaxation of the trachealis by a mechanism that involves neither beta-adrenoceptor activation nor direct reduction of the sensitivity of the intracellular contractile machinery to cytosolic free Ca2+. The action of BRL34915 may depend on the opening of K+ channels in the plasma membrane which are permeable to 86Rb+. The opening of these channels, or the effects of their opening, may be reduced by K+-channel inhibitors such as 4-aminopyridine, procaine and TEA but not by apamin.

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Electrical and mechanical effects of BRL34915 in guinea‐pig isolated trachealis

ATP-sensitive potassium channels (K(ATP) channels) are heteromeric complexes of pore-forming inwardly rectifying potassium channel subunits and regulatory sulfonylurea receptor subunits. K(ATP) channels were identified in a variety of tissues including muscle cells, pancreatic beta-cells, and various neurons. They are regulated by the intracellular ATP/ADP ratio; ATP induces channel inhibition and MgADP induces channel opening. Functionally, K(ATP) channels provide a means of linking the electrical activity of a cell to its metabolic state. Shortening of the cardiac action potential, smooth muscle relaxation, inhibition of both insulin secretion, and neurotransmitter release are mediated via K(ATP) channels. Given their many physiological functions, K(ATP) channels represent promising drug targets. Sulfonylureas like glibenclamide block K(ATP) channels; they are used in the therapy of type 2 diabetes. Openers of K(ATP) channels (KCOs), for example, relax smooth muscle and induce hypotension. KCOs are chemically heterogeneous and include as different classes as the benzopyrans, cyanoguanidines, thioformamides, thiadiazines, and pyridyl nitrates. Examples for new chemical entities more recently developed as KCOs include cyclobutenediones, dihydropyridine related structures, and tertiary carbinols.

KATP channel openers: structure-activity relationships and therapeutic potential.

Aminophylline (1-1000 mumol l-1) suppressed the spontaneous tone of guinea-pig isolated trachealis in a concentration-dependent manner. In Krebs solution containing acetylcholine (1 mmol l-1), histamine (200 mumol l-1) or K+ (120 mmol l-1) isolated trachealis muscle developed near-maximal tension. The log concentration-effect curve for aminophylline was shifted 20 fold, 3 fold and 4 fold to the right, respectively, in the presence of these spasmogens. Three K+-channel inhibitors were tested: tetraethylammonium (TEA, 8 mmol l-1) did not modify the action of aminophylline, procaine (5 mmol l-1) shifted the log concentration-effect curve for aminophylline 2 fold to the left and 4-aminopyridine (5 mmol l-1) shifted the curve 2.5 fold to the right. Intracellular electrophysiological recording showed that aminophylline 10 mumol l-1 could cause relaxation in the absence of electrical changes. Higher concentrations of aminophylline (100-1000 mumol l-1) suppressed spontaneous slow waves and hyperpolarized the trachealis cells. In the presence of procaine (5 mmol l-1) or TEA (8 mmol l-1), the hyperpolarization induced by aminophylline (1000 mumol l-1) was significantly reduced but its relaxant effect was unchanged. In trachealis skinned of its plasma membranes, tension development induced by Ca2+ (20 mumol l-1) was unaffected either by aminophylline (1000 mumol l-1) or by isoprenaline (1 mumol l-1). In studies of the efflux of 86Rb+ from muscle-rich strips of trachea, aminophylline (100-1000 mumol l-1) was without effect whereas nicorandil (100 and 1000 mumol l-1) increased the efflux rate constant. It is concluded that aminophylline does not directly reduce the sensitivity of the contractile proteins to cytosolic Ca2+. In low concentration (1-10 mumol l-1) its relaxant action is not accompanied by membrane potential change but towards the upper end of its effective concentration range, aminophylline evokes hyperpolarization. This hyperpolarization may involve the opening of K+-channels which are inhibited by procaine and (to a lesser extent) by TEA. These K+-channels may be impermeable to 86Rb+.

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Mechanical and electrical aspects of the relaxant action of aminophylline in guinea-pig isolated trachealis

1. Caffeine (10 mM)-induced relaxation of guinea-pig isolated trachealis was attenuated and converted to a small spasmogenic response on cooling to 22 degrees C. The relaxant response was restored on rewarming to 37 degrees C and was abolished by indomethacin (2.8 microM). Cooling to 22 degrees C in the presence of indomethacin revealed spasmogenic responses to caffeine which were abolished on rewarming to 37 degrees C. 2. Trachealis treated with indomethacin (2.8 microM) was repeatedly dosed with acetylcholine (ACh, 10 microM). Caffeine (1 or 10 mM), added as each ACh-induced spasm reached equilibrium, transiently augmented but then suppressed the spasm. On cooling from 37 degrees C to 12 degrees C, the increment in spasm evoked by caffeine increased relative to the spasm evoked by ACh. 3. Trachealis treated with indomethacin (2.8 microM) was repeatedly dosed with caffeine (10 mM). At 37 degrees C caffeine had little effect but it caused spasm when the tissue was cooled to 32 degrees C. Spasm amplitude increased as cooling progressed to 12 degrees C. Similar results were obtained with caffeine (1 mM). 4. At 37 degrees C, caffeine, enprofylline, 1,3,7,9-tetramethylxanthinium (TMX), theobromine, theophylline, xanthine and forskolin each caused concentration-dependent suppression of tracheal tone. Among the xanthine derivatives the rank order of potency was enprofylline greater than theophylline greater than caffeine greater than theobromine greater than xanthine greater than TMX. 5. In trachealis treated with indomethacin (2.8 microM) and maintained at 12 degrees C, the xanthines each caused concentration-dependent spasm. The rank order of potency was theobromine greater than or equal to theophylline greater than or equal to caffeine greater than or equal to enprofylline greater than xanthine greater than TMX. Forskolin was devoid of spasmogenic activity. 6. Trachealis treated with indomethacin (2.8 microM) and maintained at 12 degrees C, was repeatedly dosed with either caffeine (10 mM) or potassium chloride (KCl, 40 mM). Caffeine-induced spasm was attenuated in a Ca2+-free medium containing EGTA (2 mM), modestly at first but subsequently more profoundly. KCl did not evoke spasm at 12 degrees C but at 37 degrees C the KCl-induced spasm was virtually abolished at its first trail in the Ca2+-free, EGTA-containing medium. 7. It is concluded that caffeine, other alkylated xanthines and xanthine itself share a spasmogenic action in guinea-pig isolated trachealis which is best observed when the tissue is treated with indomethacin (2.8 microM) and maintained at 12 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)

J. Cortijo

Papers

Electrical and mechanical effects of BRL34915 in guinea‐pig isolated trachealis

Mechanical and electrical aspects of the relaxant action of aminophylline in guinea-pig isolated trachealis

The relaxant and spasmogenic effects of some xanthine derivatives acting on guinea-pig isolated trachealis muscle

Influence of some variables in the triton X-100 method of skinning the plasmalemmal membrane from guinea pig trachealis muscle