Antihistaminic and antimuscarinic effects of amitriptyline on guinea pig ileal electrolyte transport and muscle contractility in vitro.

Amitriptyline, a tricyclic antidepressant, was tested for antimuscarinic and antihistamine effects against bethanechol and histamine-stimulated contractility and secretion in the guinea pig ileum in vitro. Comparisons were made with muscarinic-receptor antagonists, as well as with H1 and H2 histamine-receptor antagonists. Amitriptyline (0.01-5.0 microM) produced a parallel rightward shift in the concentration-response curves to histamine in muscle (Ki 0.4 nM) and mucosa (Ki 450 nM). The H1-receptor antagonists pyrilamine and diphenhydramine were less potent against histamine in the muscle and more potent against histamine in the mucosa than was amitriptyline. The H2-receptor antagonist cimetidine was ineffective in the muscle and mucosa. Amitriptyline (0.1-2 microM) also produced a parallel rightward shift in the concentration-response curve to bethanechol in muscle (Ki 133 nM) and mucosa (Ki 143 nM). Against bethanechol, in both tissues, atropine and 4-diphenylacetoxy-N-methyl piperidine methiodide were more potent competitive antagonists than was amitriptyline. Pirenzepine produced a competitive blockade of bethanechol in the muscle and a noncompetitive blockade in the mucosa. The data indicate that amitriptyline exerts more potent antihistaminic effects on guinea pig ileal muscle than the mucosa but that the tricyclic drug is equipotent as an antimuscarinic in both tissues.

Exogenous ATP-stimulated calcium uptake in isolated rat intestinal epithelial cells.

ATP in the extracellular medium is known to stimulate Ca uptake into avian intestinal epithelial cells. We have now demonstrated a similar effect of ATP in mammalian intestinal epithelial cells and have further characterized this effect. Exogenous ATP increased 45Ca uptake 2-6 fold in isolated rat small intestinal epithelial cells, with a maximal effect at 1 mM and an ED50 of 290 microM. A strict structural requirement for nucleotide-stimulated 45Ca uptake was observed. ADP was much less effective than ATP and gamma-thio-ATP, and 5'-AMP, cyclic AMP, adenosine, non-adenine nucleotides, non-hydrolyzable ATP analogs and ATP analogs with ring substitutions at the 8 position were inactive. Prenylamine (100 microM) completely inhibited ATP-stimulated 45Ca uptake, while verapamil (100 microM) had only a small effect. In the intact intestine, ATP increased short-circuit current (Isc) when added to the mucosal side of the tissue. This effect was reduced by 10 microM and abolished by 100 microM prenylamine. The effect of ATP on Isc was markedly reduced in Cl-free solutions and in reduced-Ca solutions. Serosal and mucosal addition of the nonhydrolyzable ATP analog, beta, gamma-methylene-ATP, and serosal addition of ATP had little or no effect on Isc. The similarities between the effects of ATP in isolated cells and in the intact intestine suggest that the effect of ATP on Isc may be at least partially mediated through stimulation of Ca uptake into the epithelial cells.

Quantification of pepsin A activity in canine and rat gastric juice with the chromogenic substrate azocoll.

The activity concentration of pepsin may be quantified by using azocoll as a chromogenic substrate. The measured enzyme activity is constant between pH 1.2 and 3.4 and is proportional (r = 0.61) to the activity measured with hemoglobin as substrate. The activity of purified porcine pepsin is inhibited by pepstatin A with an apparent Ki of 115 nmol/L. The azocoll method is useful for measuring changes in pepsin secretion in response to pharmacological agents. For example, pepsin activity of canine gastric juice is decreased by 80% after in vivo administration of 0.5 mg of the synthetic trimethyl prostanoid Ro 22-6923 per kilogram of body weight. The method is sufficiently sensitive to measure the pepsin activity in 0.2 microL of canine gastric juice with a CV of approximately 10%, is simpler than the hemoglobin-substrate methods, and the substrate is commercially available.