Pharmacological studies with beta-adrenoreceptor blocking agents I. Effect on the smooth muscle of the taenia coli of the guinea pig

Effects of caffeine on human health

Caffeine is probably the most frequently ingested pharmacologically active substance in the world. It is found in common beverages (coffee, tea, soft drinks), in products containing cocoa or chocolate, and in medications. Because of its wide consumption at different levels by most segments of the population, the public and the scientific community have expressed interest in the potential for caffeine to produce adverse effects on human health. The possibility that caffeine ingestion adversely affects human health was investigated based on reviews of (primarily) published human studies obtained through a comprehensive literature search. Based on the data reviewed, it is concluded that for the healthy adult population, moderate daily caffeine intake at a dose level up to 400 mg day(-1) (equivalent to 6 mg kg(-1) body weight day(-1) in a 65-kg person) is not associated with adverse effects such as general toxicity, cardiovascular effects, effects on bone status and calcium balance (with consumption of adequate calcium), changes in adult behaviour, increased incidence of cancer and effects on male fertility. The data also show that reproductive-aged women and children are 'at risk' subgroups who may require specific advice on moderating their caffeine intake. Based on available evidence, it is suggested that reproductive-aged women should consume </=300 mg caffeine per day (equivalent to 4.6 mg kg(-1) bw day(-1) for a 65-kg person) while children should consume </=2.5 mg kg(-1) bw day(-1).

Studies on the estrogenic activity of a coffee extract

Ethyl ether extracts derived from coffee were tested for in vitro estrogenic and in vivo uterotropic activities. Coffee extracts, unlike tea and cocoa, were found to actively compete with 17 beta-estradiol for uterine cytosol binding sites. The biologically active fractions possessed an unique ultraviolet absorbance spectrum that excluded them from containing flavonoid, coumestan, or resorcyclic acid lactone constituents. Coffee extracts administered to immature female mice for 3 d in feeding studies displayed significant (p less than 0.05) uterotropic responses, which were similar to results obtained in mice treated with a standard 17 beta-estradiol dose. Additional studies in mice disclosed that coffee extracts did not reduce the uterotropic effect normally induced by 17 beta-estradiol when administered simultaneously with estradiol. The complete estrogenic effects of coffee constituents, coupled with their failure to inhibit a biological response evoked by estradiol, strongly suggest that coffee contains constituent(s) that are weakly estrogenic.

Responsiveness to noradrenaline of longitudinal and circular smooth muscles of cat caecum during the postnatal period

1. The changes in the responsiveness of the isolated longitudinal and circular smooth muscles from cat caecum to adrenergic agents during the postnatal period were studied by the effect of noradrenaline (0.01-100 mumol) on the smooth muscle tone and phasic contractions. 2. Noradrenaline (10 mumol) contracted the preparations of longitudinal and circular muscles isolated from cats between 15 and 60 days postnatal via stimulatory alpha 1-adrenoceptors. 3. Noradrenaline applied cumulatively (0.01-100 mumol) produced a concentration-determined but age-independent inhibitory effect on the phasic contractions of longitudinal muscle and exercised a concentration- and age-determined effect on the phasic activity of circular muscle. 4. The adrenoceptors of cat caecum are not differentiated until 60 days postnatal.

Postnatal development of responsiveness to adrenergic agents of longitudinal and circular smooth muscles from cat terminal ileum

The changes in the responsiveness of the isolated longitudinal and circular smooth muscles from cat terminal ileum to adrenergic agents during the postnatal period were studied by the effect of noradrenaline (0.01-100 mumol) on the smooth muscle tone and phasic contractions. Cumulatively applied noradrenaline (0.01-10 mumol) decreased the longitudinal muscle tone without age-dependent differences via inhibitory alpha 1-adrenoceptors. Noradrenaline decreased the phasic contraction amplitude of longitudinal muscle via inhibitory alpha 1-adrenoceptors. Cumulatively applied noradrenaline (0.01-10 mumol) exerted an age-dependent effect on the circular muscle tone via stimulatory alpha 1-adrenoceptors. alpha 1-adrenoceptors continue to develop functionally at 60 days postnatal. Noradrenaline decreased the phasic contraction amplitude of the circular muscle and exerted a stimulant effect on the tone which suggested an existence of two alpha 1-adrenoceptor subtypes.

Effects of 3,4-diaminopyridine and tetraethylammonium on the pre- and post-junctional alpha-adrenoceptor mediated inhibitory actions of noradrenaline in the guinea-pig ileum

The effects of potassium channel blockers, 3,4-diaminopyridine (DAP) and tetraethylammonium (TEA) were studied on the pre- and post-junctional alpha 2-adrenoceptor mediated effects of noradrenaline in the guinea-pig proximal ileum. Both DAP (4 to 500 mumol l-1) and TEA (0.3 to 3 mmol l-1) transiently increased the basal tension of the ileum. However, DAP also increased the amplitude of the smooth muscle twitches evoked by transmural nerve stimulation, whereas TEA marginally depressed them. Atropine (2 mumol l-1) antagonized the contractions induced by DAP but did not affect the similar effects of TEA. On the other hand, DAP restored the smooth muscle twitches depressed by atropine, while TEA did not. DAP, in a concentration-dependent manner, reduced or abolished the prejunctional inhibitory alpha 2-adrenoceptor mediated effect of noradrenaline, whereas TEA (up to 3 mmol l-1) was almost ineffective. The postjunctional inhibitory alpha 2-adrenoceptor mediated effect of noradrenaline was attenuated even at the smallest TEA concentration used (0.3 mmol l-1) and its postjunctional stimulatory alpha 1-adrenoceptor mediated effect was unmasked. However, DAP, was only marginally effective, even at the highest concentrations used (100 and 500 mumol l-1). From these results it would appear that in both the pre- and post-junctional inhibitory alpha 2-adrenoceptor mediated actions of noradrenaline in the guinea-pig ileum the primary step might be an increased potassium conductance. However, the potassium channels on the neuronal and the smooth muscle membrane have different sensitivities to DAP and TEA.

Homogeneous and non-homogeneous distribution of inhibitory and excitatory adrenoceptors in the longitudinal muscle of the guinea-pig ileum

1 The effects of adrenoceptor agonists and antagonists on spontaneous and evoked membrane activities of longitudinal muscle cells from different parts of the guinea-pig ileum were observed, using microelectrode methods.2 Isoprenaline inhibited the generation of spikes in cells in the terminal (0-3 cm from the ileocaecal valve) and proximal (more than 50 cm from the ileocaecal valve) regions of the ileum, with no change on the membrane potential and ionic conductance of the membrane. These actions of isoprenaline were abolished by propranolol.3 Noradrenaline and phenylephrine depolarized the membrane and increased both the spike frequency and ionic conductance of cell membranes of the terminal ileum, whereas noradrenaline and clonidine hyperpolarized the membrane, increased the ionic conductance of the membrane and inhibited the spontaneously generated spikes from cells of the proximal ileum. The excitatory effect of phenylephrine in the cells of the proximal ileum and the inhibitory effect of clonidine on cells of the terminal ileum were less pronounced.4 The excitatory actions of noradrenaline or phenylephrine were antagonized by prazosin and phentolamine, but not by yohimbine, whereas the inhibitory actions of noradrenaline or clonidine were antagonized by yohimbine and phentolamine but not by prazosin.5 The cholinergic e.j.ps evoked by field stimulation to the tissue were not affected by isoprenaline or phenyleprine but were inhibited by noradrenaline and clonidine, in both the terminal and proximal regions of the ileum. These actions of noradrenaline and clonidine were antagonized by yohimbine but not by prazosin.6 The results indicate that in the myenteric plexus and longitudinal muscle tissues of the guinea-pig ileum there are prejunctional inhibitory (alpha(2)), postjunctional inhibitory (alpha(2) and beta) and postjunctional excitatory (alpha(1)) adrenoceptors. The homogeneous distributions of prejunctional alpha(2)- and postjunctional beta-adrenoceptors in the ileum are responsible for inhibitions of cholinergic excitatory junction potentials (e.j.ps) and spontaneous spike activities, respectively. The density of distribution of the postjunctional alpha(1)-adrenoceptors is higher in the terminal than in the proximal regions, and these distributions are reversed in the case of the postjunctional alpha(2)-adrenoceptors. The postjunctional alpha(1)-adrenoceptors are probably responsible for the membrane depolarization and alpha(2)-adrenoceptors for the hyperpolarization induced by catecholamines.

Distribution and types of adrenoceptors in the guinea-pig ileum: the action of alpha- and beta-adrenoceptor agonists

1 Segments of guinea-pig ileum and the myenteric plexus-longitudinal smooth muscle preparation were used for a study of the actions of adrenaline, noradrenaline, isoprenaline, ephedrine and phenylephrine on the responses of coaxially stimulated ileum at different distances from the ileocaecal valve.2 The responses of the ileum to electrical stimulation were suppressed by adrenaline, nonadrenaline and ephedrine, while phenylephrine and isoprenaline inhibited them only partially.3 The twitch inhibition elicited by these adrenoceptor agonists was the same at all distances from the ileocaecal valve. There was no significant difference between their cumulative and non-cumulative concentration-response curves.4 Smooth muscle relaxation was induced only by isoprenaline and contraction only by phenylephrine at all distances from the ileocaecal junction. Adrenaline and noradrenaline evoked smooth muscle contraction in the terminal (0 to 20 cm), a concentration-dependent, biphasic response in the intermediate part (21 to 50 cm) and a relaxation in the proximal ileum (> 50 cm from the ilecocaecal valve). Ephedrine did not change significantly the smooth muscle tension in the terminal and the intermediate segments and induced smooth muscle relaxation in the proximal ones.5 Ouabain and a potassium-free solution did not appear to influence the prejunctional action of noradrenaline nor the amplitude of smooth muscle relaxation in the proximal ileum, whereas the concentration-contractor response curves were significantly depressed and shifted to the right by ouabain and in a potassium-free solution.6 The brief initial (phasic) contraction induced by acetylcholine was not influenced during the sustained increase or decrease in tension induced by catecholamines. On the contrary, the stimulatory catecholamine actions disappeared or were changed to smooth muscle relaxation by acetylcholine pretreatment. Potassium chloride pretreatment did not change the character of the adrenoceptor agonist action of the agonists studied.7 Since there is a similar prejunctional action at all distances from the ileocaecal valve and a different postjunctional effect of the adrenoceptor agonists at different distances from the ileocaecal junction, it could be suggested that in the guinea-pig ileum there are at least two alpha-adrenoceptors (inhibitory prejunctional-alpha(2), stimulatory postjunctional-alpha(1)), an inhibitory postjunctional beta-adrenoceptor and an as yet uncharacterized inhibitory postjunctional receptor.8 Based on the specific postjunctional action of phenylephrine and the prejunctional action of ephedrine in the guinea-pig ileum, these drugs could be used with success as ;specific' alpha(1)- and alpha(2)-adrenoceptor stimulants.