Ca2+ channel blockers interact with alpha 2-adrenergic receptors in rabbit ileum.

An interaction between Ca2+ channel blockers and alpha 2-adrenergic receptors has been demonstrated in rabbit ileum by studying the effect of clonidine on active electrolyte transport, under short-circuited conditions, in the presence and absence of several Ca2+ channel blocking agents. Clonidine, verapamil, diltiazem, cadmium, and nitrendipine all decrease short-circuit current and stimulate NaCl absorption to different extents with clonidine having the largest effect. Exposure to verapamil, diltiazem, and cadmium inhibited the effects of clonidine on transport, whereas nitrendipine had no such effect. Verapamil, diltiazem, and cadmium, but not nitrendipine, also decreased the specific binding of [3H]alpha 2-adrenergic agents to a preparation of ileal basolateral membranes explaining the observed decrease in the transport effects of clonidine. The effective concentrations of the Ca2+ channel blockers that inhibited the effects of clonidine on transport were fairly similar to the concentrations needed to inhibit its specific binding. The displacement of clonidine by calcium channel blockers is ascribed to a nonspecific effect of these agents, although the possibility that their effects are exerted via their binding to the calcium channels is not excluded.

Effects of calcium antagonist TMB-8 on active Na and Cl transport in rabbit ileum.

The effects of 3,4,5-trimethoxybenzoate 8-(N,N-diethylamino)octyl ester (TMB-8), an agent that traps calcium within intracellular stores, were studied on active electrolyte transport in rabbit ileum under basal conditions and after altering transport by increasing the intracellular cAMP content or by exposure to two agonists that act by altering intracellular Ca2+ (carbachol and serotonin). TMB-8 decreased the ileal short-circuit current and increased active Na and Cl absorption by increasing the mucosal-to-serosal Na and Cl fluxes. These effects were reversed by increasing the bathing solution Ca2+ to 4 mM, a concentration that itself did not alter basal ileal transport. The maximum glucose- and amino acid (alanine)-induced increase in Na absorption in the ileum was not affected by TMB-8. The effects on basal transport of TMB-8 were not associated with a change in 45Ca2+ entry across the ileal serosal surface. TMB-8 did not alter cAMP-induced secretion, as judged by its lack of effect on the increase in short-circuit current caused by 8-bromo-cAMP (10(-4) M). TMB-8 totally prevented the transport effects of carbachol but did not inhibit the effects of serotonin. These data suggest a role for intracellular Ca2+ in regulation of basal ileal Na and Cl transport but not in cAMP-induced secretion. There appear to be several pools of intracellular Ca2+ involved in neurohumoral effects on active electrolyte transport.

Dantrolene and basal ileal sodium and chloride transport: involvement of calcium stores.

The effect of dantrolene on active ion transport in rabbit ileum was determined using the Ussing chamber short-circuiting technique. Dantrolene prevents the release of calcium from intracellular stores in skeletal muscle and was used to probe the role of intracellular calcium stores in intestinal ion transport. A saturated solution of dantrolene (approx 25 microM) decreased ileal short-circuit current and potential difference, increased conductance and mucosal-to-serosal and net Na and Cl fluxes, but did not alter serosal-to-mucosal Na and Cl fluxes. The dantrolene stimulation of active Na and Cl absorption was specific since it did not alter glucose-dependent Na absorption, transport changes caused by Ca2+ ionophore A23187, or the increase in short-circuit current caused by dibutyryl cAMP or theophylline. These effects were associated with an increase in total ileal calcium content and a decreased rate of 45Ca2+ efflux without any change in 45Ca2+ influx from the serosal or mucosal surfaces. These findings are consistent with an effect of dantrolene to stimulate active ileal Na and Cl absorption by a mechanism involving lowered cytosol Ca2+ levels and compatible with trapping calcium in intracellular stores. It thus appears as if intracellular calcium stores have an important role in the control of basal ion transport in the intestine.

Dopamine stimulation of active Na and Cl absorption in rabbit ileum: interaction with alpha 2-adrenergic and specific dopamine receptors.

The effects of dopamine on active intestinal ion transport have been evaluated. An epithelial sheet preparation of rabbit ileum was used in vitro with the Ussing chamber-voltage clamp technique. Dopamine, in the presence of 1 mM ascorbic acid, added to the serosal bathing solution caused a dose-dependent decrease in short-circuit current, with a half-maximal effect at 1.2 muM and maximal effect of -50 muA/cm(2) at 50 muM; dopamine decreased the potential difference, and increased the conductance and net Na and net Cl absorption. There was no effect on the residual ion flux. Dopamine did not alter the change in short-circuit current caused by mucosal glucose (10 mM) or serosal theophylline (10 mM). Mucosal dopamine had no effect. The effect of dopamine on short-circuit current was inhibited by the dopamine antagonists haloperidol and domperidone and the alpha(2)-adrenergic antagonist yohimbine; there was no effect of the alpha(1)-antagonist prazosin and the beta-antagonist propranolol. In addition, the alpha(2)-adrenergic agonist clonidine, but not the alpha(1)-agonist methoxamine caused a dose-dependent decrease in short-circuit current. The ileal effects of dopamine did not occur via conversion into norepinephrine or release of norepinephrine from the peripheral nerves since "peripheral sympathectomy" with 6-hydroxydopamine did not alter the dopamine-induced change in ileal short-circuit current. The dopamine effects were not associated with a change in basal ileal cyclic AMP content but were associated with a decrease in total ileal calcium content as measured by atomic absorption spectrometry and as estimated by (45)Ca(++) uptake. The decrease in calcium content could be attributed to a dopamine-induced decrease in (45)Ca(++) influx from the serosal surface. Because of the presence of dopamine in ileal mucosa and these effects on ileal electrolyte transport, it is possible that dopamine may be involved in the physiologic regulation of active intestinal electrolyte absorption.