Comparison of the effects of caffeine and procaine on noradrenergic transmission in the guinea-pig mesenteric artery

Effect of caffeine contained in a cup of coffee on microvascular function in healthy subjects

Recent epidemiological studies have demonstrated that coffee drinking is associated with reduced mortality of cardiovascular disease. However, its precise mechanisms remain to be clarified. In this study, we examined whether single ingestion of caffeine contained in a cup of coffee improves microvascular function in healthy subjects. A double-blind, placebo-controlled, crossover study was performed in 27 healthy volunteers. A cup of either caffeinated or decaffeinated coffee was drunk by the subjects, and reactive hyperemia of finger blood flow was assessed by laser Doppler flowmetry. In an interval of more than 2 days, the same experimental protocol was repeated with another coffee in a crossover manner. Caffeinated coffee intake slightly but significantly elevated blood pressure and decreased finger blood flow as compared with decaffeinated coffee intake. There was no significant difference in heart rate between caffeinated and decaffeinated coffee intake. Importantly, caffeinated coffee intake significantly enhanced post-occlusive reactive hyperemia of finger blood flow, an index of microvascular endothelial function, compared with decaffeinated coffee intake. These results provide the first evidence that caffeine contained in a cup of coffee enhances microvascular function in healthy individuals.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Pharmacological distinction of the hyperpolarization response to caffeine and acetylcholine in guinea-pig coronary endothelial cells

Membrane potential changes in endothelial cells in response to caffeine and acetylcholine (ACh) were recorded with microelectrodes from an intact endothelium preparation from the guinea-pig coronary artery. Caffeine induced a transient hyperpolarization of the membrane in a concentration-dependent manner. The hyperpolarization was inhibited by removal of Ca2+ from the bathing medium and by ryanodine (20 microM). It was not affected by 3,4,5-trimethoxybenzoic acid 8-(diethylamino) octyl ester hydrochloride (TMB-8, 10 microM) or neomycin (5 mM). ACh induced a sustained hyperpolarization in endothelial cells. At concentrations that caused no significant effects on the caffeine response, TMB-8 and neomycin inhibited hyperpolarization induced by ACh. Ryanodine did not inhibit the response to ACh. The ACh-induced hyperpolarization was also inhibited by caffeine in a concentration-dependent manner. Results from the present study suggest that hyperpolarizations induced by caffeine and ACh are mediated by separate Ca2+ pools.

Calcium dependency of the endothelium-dependent hyperpolarization in smooth muscle cells of the rabbit carotid artery

1. In smooth muscle cells of the rabbit carotid artery, ACh (greater than 10(-8) M) generated a hyperpolarization with two components (transient followed by sustained), only in the tissues with an intact endothelium. There were no detectable changes in the membrane potential, as elicited by ACh (up to 10(-5) M) in tissues with no endothelium or in the presence of atropine (10(-6) M). 2. Reduction of [Ca2+]o inhibited the sustained component which was not apparent in [Ca2+]o below 0.16 mM. In Ca2+-free (EGTA-containing) solution, the generation of the transient component of the hyperpolarization remained sustained but with a substantially reduced amplitude. 3. Procaine (greater than 10(-6) M) inhibited the ACh-induced hyperpolarization in a concentration-dependent manner, and at a concentration of procaine (10(-3) M) which caused substantial depolarization of the membrane, no detectable change was elicited by ACh. 4. Caffeine (10(-6)-10(-3) M) produced a transient hyperpolarization, independent of the presence or absence of the endothelium, and inhibited the sustained component of the ACh-induced hyperpolarization more so than the initial component. 5. A23187 (greater than 10(-8) M) hyperpolarized the smooth muscle membrane in a concentration-dependent manner, and this hyperpolarization was not generated in Ca2+-free solution or in the absence of endothelial cells. 6. In intact tissues, pre-treatment with A23187 resulted in a reduction of the subsequently generated ACh-induced hyperpolarization, in an irreversible manner. 7. It would thus appear that in the rabbit carotid artery, the endothelium-dependent hyperpolarization induced by ACh has Ca2+-dependent and Ca2+-independent components, and each may be related to the increase in endothelial [Ca2+]i by release from the intracellular store and by influx from the extracellular medium, respectively. The increased [Ca2+]i would trigger a release of an endothelium-derived hyperpolarizing factor (EDHF) from the endothelial cells.

Pre- and post-junctional effects of adenosine triphosphate on noradrenergic transmission in the rabbit ear artery

1. The effects of adenosine triphosphate (ATP), 5'-adenylylimidodiphosphate (AMP-PNP) or alpha,beta-methylene ATP (mATP) on the excitatory junction potential (e.j.p.) and slow depolarization evoked by perivascular nerve stimulation were studied in smooth muscle cells of the rabbit ear artery. 2. ATP (above 10(-6) M), AMP-PNP (above 10(-6) M) and mATP (above 10(-8) M) transiently (10-15 min) depolarized the membrane. The membrane remained depolarized after prolonged exposure (over 20 min) to ATP (above 3 X 10(-5) M), AMP-PNP (above 10(-5) M) or mATP (above 3 X 10(-8) M). 3. ATP (above 10(-5) M), AMP-PNP (above 5 X 10(-6) M) or mATP (above 3 X 10(-8) M) decreased the membrane resistance. Increasing the external K+ concentration (K+o) to 10.1 mM also decreased the membrane resistance, with an associated depolarization. 4. ATP (10(-6)-5 X 10(-5) M) or AMP-PNP (over 10(-6) M) transiently decreased and then increased amplitudes of the e.j.p. and of the slow depolarization, the latter component increasing more than the former. 5. Depolarization of the membrane by 10.1 mM-K+o solution or mATP (10(-7) M) decreased the amplitude of e.j.p.s, with no change in the facilitation, and the slope of the relationship between amplitude of e.j.p. and that of slow depolarization decreased with mATP but not with 10.1 mM-K+o solution. 6. The outflows of noradrenaline and 3,4-dihydroxyphenylglycol (DOPEG) induced by perivascular nerve stimulation increased with ATP (above 10(-6) M) or AMP-PNP (above 10(-5) M), while there was no change with mATP (10(-8)-10(-5) M) or 10.1 mM-K+o solution. 7. Pre-treatment with mATP inhibited the ATP-induced increase in the outflow of noradrenaline and DOPEG, and also the ATP-induced enhancement of the amplitude of the e.j.p. 8. Therefore ATP and AMP-PNP have predominantly excitatory actions on both pre- and post-junctional membranes, while mATP has an excitatory action on the post-junctional membrane but antagonizes the facilitatory action of ATP on release of noradrenaline from the nerve terminal.