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PubMed1981

An in vitro quantitative analysis of the alpha adrenoceptor partial agonist activity of dobutamine and its relevance to inotropic selectivity.

In rat anococcygeus muscle, dobutamine produced concentration-related submaximal contractions which were antagonized competitively by phentolamine (pKB = 8.3) and dobutamine antagonized norepinephrine-induced contractions in a competitive manner with an equilibrium dissociation constant for the alpha adrenoceptor of 20 nM (pKB = 7.7). Therefore, dobutamine satisfied criteria for a partial agonist of alpha adrenoceptors having an affinity for alpha adrenoceptors 25 times that of norepinephrine (pKA = 6.3) in this tissue. An estimate of the relative efficacy of dobutamine showed one-fortieth the the efficacy of norepinephrine at the alpha adrenoceptors. Dobutamine contracted rabbit aorta and produced concentration-related relaxations at 1000 times greater concentrations after alkylation of alpha adrenoceptors by phenoxybenzamine. In noncontracted canine saphenous vein, dobutamine had no visible agonist activity but did produce contractions after propranolol. In partially contracted saphenous vein, dobutamine produced a small contraction which was converted to a propranolol-sensitive relaxation of tone after phentolamine. Dobutamine was a full beta adrenoceptor agonist in guinea-pig trachea under spontaneous tone but a partial agonist after strong contraction by bethanechol. This allowed measurement of the pKB of dobutamine at beta adrenoceptors (pKB = 5.35) and estimation of efficacy at beta adrenoceptors relative to isoproterenol (eDob/eIso = 1/20). No evidence for beta adrenoceptor selectivity was found in studies of potency ratios and relative efficacy using isoproterenol for comparison. Dobutamine showed a slight (2-fold) selectivity for inotropy in vitro when compared to isoproterenol in guinea-pig right and left atria. This selectivity was removed by phentolamine suggesting a cardiac alpha-like adrenoceptor effect; this finding was confirmed in propranolol-treated guinea-pig left atria. These results are discussed in terms of the in vivo effects of dobutamine and its use as a tool for classification of beta adrenoceptors, particularly the putative presynaptic beta adrenoceptor.

Adrenergic alpha-Agonists↗
PubMed1981

Cyclic guanosine 3':5'-monophosphate accumulation and 45Ca-uptake by rat superior cervical ganglia during preganglionic stimulation.

Repetitive preganglionic nerve stimulation increases cyclic guanosine 3':5'-monophosphate (cGMP) content in rat superior cervical ganglia by a mechanism requiring Ca++ but resistant to blockade by cholinergic receptor antagonists. Similarly, 45Ca-uptake during prolonged preganglionic nerve stimulation is unaffected by hexamethonium or atropine. These findings indicate that nerve stimulation increases cGMP accumulation and 45Ca-uptake by a noncholinergic mechanism Substance P, met-enkephalin and luteinizing hormone-releasing factor have little or no effect on cGMP content. By contrast, bethanechol causes a 3-fold increase in cGMP content and postganglionic cell firing. Thus, as reported by others, muscarinic receptor activation increases ganglionic cGMP[. 4-Aminopyridine causes an increase in cGMP of resting ganglia that requires Ca++ and the nerve terminal is blocked by tetrodotoxin but unaffected by atropine or hexamethonium. Ouabain also increases ganglionic cGMP content by a process that requires Ca++ and the nerve terminals. Like preganglionic nerve stimulation, 4-aminopyridine and ouabain cause cGMP accumulation in the nerve terminals or in the ganglion cells as a consequence of releasing a noncholinergic transmitter. The uptake of Ca++ by ganglion cells is not an adequate stimulus for cGMP accumulation because the nicotinic receptor agonist dimethylphenylpiperazinium increases 45Ca-uptake but has no effect on cGMP formation in ganglia.

4-Aminopyridine↗
PubMed1981

A review of the animal pharmacology of ranitidine--a new, selective histamine H2-antagonist.

The animal pharmacology and metabolism of ranitidine have been reviewed. Experiments using guinea-pig isolated right atrium and ileum preparations have shown that ranitidine is a selective, potent, and competitive histamine H2-receptor antagonist. In the conscious dog gastric acid secretion induced by histamine, pentagastrin, bethanechol and food was inhibited by ranitidine at doses 4 to 12 times lower than equi-effective doses of cimetidine. Ranitidine inhibited the formation of aspirin-induced gastric lesions, both in the presence and absence of gastric acid. Unlike cimetidine, ranitidine neither possessed anti-androgenic activity, nor did it inhibit the mixed function oxygenase metabolizing enzyme system in the liver. Ranitidine has been recommended for clinical trial in the treatment of peptic ulcer disease.

Administration, Oral↗