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

Abstract

SUMMARY1. Shortening and rate of loss of 42K 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 ofatropine caused equal maximal shortenings ofthe muscle, but unequal maximal increases in 42K efflux: maximal 42K effluxes obtainable in response to raised [K+]o and histamine were about Z and 1 respectively of the maximal efflux in response to carbachol. In the absence of atropine the increase in 42K 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 H2-receptor blocker (cimetidine), the maximum 42K efflux produced by histamine was not increased. An analogue ofhistamine reputed to resist metabolic degradation did not produce a larger increase in 42K efflux than histamine. The smaller maximal effect of histamine on 42K 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 42K efflux was about 1: 1-3 for raised [K+]. 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 42K 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 42K 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.

Maintained contractions of rat uterine smooth muscle incubated in a Ca2+‐free solution

“…However, the position of the equilibrium potential for acetylcholine (close to zero potential) presumably reflects the balance between Na+-inward and K+-outward movements (Bolton & Clark, 1981).…”

Mechanism of carbachol‐evoked contractions of guinea‐pig ileal smooth muscle close to freezing point

“…When a steady resting level of tension in the high potassium solution was achieved, stimulation with 100 JAM carbachol at 37°C induced an additional increase in tension (Figure 3a). This was followed by a large relaxation upon returning to drugfree solution as observed previously (Bolton & Clark, 1982). In 2 of 13 preparations the tension actually rose again and then returned to a lower level demonstrating that the sustained phase of the contraction was not completely abolished in the depolarized tissue.…”

Palytoxin-induced K+ efflux from ileal longitudinal smooth muscle of the guinea-pig.

“…CCh induces contraction and increases in K+ efflux in the guinea-pig ileal longitudinal muscle (20). As the CCh-induced K+ efflux was markedly inhibited by atropine (10-6 M), Cat+o removal and verapamil (10-5 M), the CCh-induced K+ efflux seems to be due to receptor-linked Cat+ influx.…”

“…In guinea-pig ileal longitudinal smooth muscle, carbachol (CCh) also produced an increase in K+ efflux, membrane depolariza tion and a contraction (20). In this experi ment, we compared the properties of K+ efflux induced by PTX and CCh in guinea-pig ileal longitudinal smooth muscle.…”