The history of how we reached the goal of determining the mechanism of vasodilatation caused by non-adrenergic, non-cholinergic nerve stimulation in cerebral arteries was traced. We concluded from this project that electrical and chemical (by nicotine) stimulations evoke an increased influx of Ca2+ into nerve terminals and activate nitric oxide (NO) synthase, resulting in the synthesis and release of NO that stimulates the guanylate cyclase in smooth muscle, thereby causing the accumulation of cyclic GMP and eliciting muscle relaxation. Reviewed also are the neurally-induced inhibitory responses of extracranial arteries, intestines, etc. with respect to NO.
The wide distribution of serotonin (5-HT) receptors and 5-HT-mediated effects suggest that 5-HT has important physiological and/or pathophysiological roles in the peripheral nervous system. Recent studies about the localization and release of 5-HT in the peripheral nervous system suggest that 5-HT likely plays a functional role in regulating discharge of autonomic neurons and modulating sensory afferent neurons. The electrophysiological approach has the merit of measuring a direct effect of 5-HT on its receptor. In this review, much of our described work focuses on effects mediated via 5-HT3-receptors on respiration and circulation using electrophysiological techniques, and we consider whether these actions of 5-HT have physiological and/or pathophysiological significance.
Since the existence of β3-adrenoceptors in various organs has been established, it is necessary to re-evaluate the subtypes of β-adrenoceptors in cardiac muscle. We have demonstrated the possible existence of three subtypes of β-adrenoceptors: β2-adrenoceptors, high-affinity β1-adrenoceptors (the so-called β1-adrenoceptors) and the low-affinity β1-adrenoceptors (akin to β3-adrenoceptors) in canine cardiac muscle, which are coupled with increases in myocardial cyclic AMP content and positive inotropic effects. Cyclic AMP generated by the activation of the high-affinity β1-adrenoceptors seems to be coupled with the positive inotropic effect much more effectively than via β2- or low-affinity β1-adrenoceptors. Stimulation of β2-adrenoceptors or the low affinity β1-adrenoceptor is causally related to the development of tolerance and to the adverse effects of nonselective β-full agonists. It is conceivable that with denopamine, unlike with isoproterenol, tolerance can hardly develop, and there are no adverse effects resulting from its partial agonistic property and its selectivity for the high-affinity β1-adrenoceptors. The selective stimulation of the high-affinity β1-adrenoceptors would be beneficial for the management of mild congestive heart failure. In contrast, stimulation of the low-affinity β1-adrenoceptors by endogenous catecholamines or nonselective β-agonists will contribute to deteriorating hemodynamic, symptomatic and prognostic consequences in patients with congestive heart failure.
Endothelin-1 (ET-1), ET-2 and ET-3 are 21-amino acid peptides that act to stimulate contraction of many smooth muscle tissues including blood vessels, uterus, bladder, and intestine. In the case of each ET, the 21-amino acid bioactive form is produced from each precursor (referred to as big ET) via specific conversion between Trp-Val (ET-1, ET-2) or Trp-Ile (ET-3). Northern blot analysis of the mRNA for the ET isoform as well as determination of the peptide itself revealed that they are independently expressed in various tissues, indicating that each isoform may play separate physiological or pathophysiological roles. However, the vascular endothelial cells appear to produce only ET-1. Since ET-1 was first discovered, the majority of investigations so far reported have been concerned with this particular peptide. This review will focus on ET-1 with respect to the regulation of its biosynthesis in endothelial cells and its precise pharmacological actions on the cardiovascular system including the cerebral, coronary and renal circulations. Sub-classification of ET receptors was made from molecular biological, biochemical as well as pharmacological points of view. The signal transduction for mechanisms of action was also explained in detail.
The development of vasodilator drugs that open the K+ channels in blood vessels has been of great academic and practical interest. The discoveries of the ATP-sensitive K+ channel and the glibenclamide-sensitive K+ channel have promoted these interests. In relation to this channel, the cardioprotective effectiveness of a K+ channel opener (Aprikalim) in doses that did not change haemodynamics or collateral blood flow were demonstrated in infarct dog heart. The effects were antagonized by glibenclamide. Thus, ATP-sensitive K+ channels seem to play an important role in this effect. Clinical evaluations of the K+ channel openers are reviewed. The hypotensive effects of the drugs are well-recognized. At present, however, the clinical usefulness of K+ channel openers has not been accepted widely, because of their side-effects including reflex tachycardia, edema, flushing and headache. An approach to reduce these side-effects is critical if these K+ channel openers are to be used as good hypotensive drugs. The K+ channel opener nicorandil has been evaluated as a highly effective antianginal drug. It seems likely that the clinical benefits of nicorandil result from both its K+ channel opening properties and its ability to stimulate smooth muscle guanylate cyclase. Clinical data on the pure-selective K+ channel opener cromakalim (lemakalim) as an antianginal drug is limited; However, on the basis of the vasodilator profile of this drug, it is expected to be useful for this purpose. The application of K+ channel openers to treat other disorders such as bladder instability is limited because of its hypotensive action.
The importance of Ca2+ for the transmission of information inside living cells is well-recognized. The mobilized Ca2+ binds to the intracellular Ca2+ binding proteins to transmit the Ca2+ signal. Calmodulin is the most important Ca2+ binding protein to exert pleiotropic effects on various cellular functions by activating multiple enzymes. To determine the physiological significance of the Ca2+ messenger system, specific inhibitors are quite essential. We have been originally developing novel and selective inhibitors, and analyzing the several processes involved in information flow in the Ca2+-dependent regulatory system of cell functions. The aim of this review is to introduce our research strategy for the production of selective inhibitors towards calmodulin-dependent enzymes and to discuss recent progress in the understanding of the multiple calmodulin-dependent pathways that mediate the action of exogenous physiological stimuli. We summarize much of the pharmacological evidence that has led to our current knowledge of calmodulin-regulated cell functions, with emphasis on aspects that may be relevant to drug design. These H-series inhibitors are one of the most powerful tools as molecular probes for pharmacological approaches, and will shed light on the physiological significance and molecular mechanisms of the calmodulin-dependent pathways in various cell functions.
To develop a new concept of central acting drugs, the modulation of brain Ca2+ flux must be considered as one of the important factors. This is because excessive Ca2+ influx to neuronal cells damages or kills these cells, and also because abnormal intracellular Ca2+ concentrations induce several types of mental disorders. Recently, both pre-clinical and clinical studies indicated that some Ca2+ channel blockers (Ca antagonists) will be useful for the treatment of grand mal, manic depressive insanity, panic disorder and anxiety. Furthermore, it has been estimated by animal studies and clinical pharmacology that ischemia-induced neuronal death can be prevented by the treatment with a Ca antagonist. However, the latter data, especially, has been mainly explained by pharmacological effects on the cerebrovascular system, not because of possible direct central actions. To invoke the notion of direct central action, it must be assumed that Ca antagonists might pass the blood-brain barrier (BBB). This potentiality that some Ca antagonists (i.e., flunarizine, nicardipine, nimodipine, etc.) can pass the BBB has been initially explored. If substantiated, such direct central effects of Ca antagonists may explain both the psychotropic effects and neuronal protection by these agents. To investigate the actual therapeutic effects of Ca2+ antagonists on psychotropic disorders and neuronal death, a suitable animal model and reasonable methods and criteria must be established. Then, both preclinical and clinical studies can be expected to relate to atypical central acting drugs modulating the brain Ca2+ channels, and also to the development of new pharmacological properties of Ca2+ antagonists.
The existence of multiple isozymes of cyclic nucleotide phosphodiesterase (PDE) in many tissues including the heart has been demonstrated. Five isozyme families, each composed of several subtypes and having different tissue and subcellular distributions, have been characterized. Selective inhibitors of PDE III (cGMP-inhibited PDE) elevates the cAMP level which mediates positive inotropic actions with compartmentation of cAMP related to cardiac cell particulate structures. Both cardiac cytosolic and particulate PDE III were potently and selectively inhibited by the new cardiotonic agents competitively with respect to cAMP, except for vesnarinone. There might be at least two subtypes of PDE III, and vesnarinone may be a selective subtype inhibitor of PDE III in human heart. It was also reported that vesnarinone was beneficial in treating patients with congestive heart failure. Moreover, selective inhibitors of PDE III with ancillary properties such as calcium sensitization may prove to be more useful drugs for the treatment of heart failure.
Since the use of cardiac glycosides for heart failure therapy is limited by their narrow margin of safety, numerous efforts have been made to find and develop novel cardiotonic agents that are superior to the cardiotonic glycosides. Positive inotropic drugs acting on β-adrenoceptors and inhibitors of cAMP phosphodiesterase have been extensively studied for the treatment of patients with heart failure. The main mechanism of these agents is elevation of cAMP tissue levels. Furthermore, Ca sensitizers such as sulmazol, pimobendan, MCI-154, END 53998 and DPI 201-106 are of interest, since such a mechanism of action may be beneficial for the failing heart. Recently, cardiotonic substances with a novel mechanism of action such as gingerol and xestoquinone have been isolated from natural sources. Natural products, purealin, goniodomin and okadaic acid, have proven to be valuable pharmacological tools for studies on cardiac muscle contraction.
Thrombus formation is initiated by platelet aggregation, followed by activation of the blood coagulation system, so that anti-thrombotic drugs can be classified into anti-platelet drugs, anti-coagulation drugs and fibrinolytic drugs. A variety of thrombus models in animals have been reported. Microthrombus formation in arterioles in the microcirculation is useful for analyzing the nature of the thrombus and the process of thrombus formation. Platelet aggregation can be divided into two types: (a) reversible aggregation (Rev-Aggr), in which platelets do not release their granular contents and are able to return to the resting discoid shape and (b) irreversible aggregation (Irrev-Aggr), in which platelets release their granular contents. Irrev-Aggr is inhibited by aspirin and not by PGI2 at the maximum aggregation, whereas Rev-Aggr is not inhibited by aspirin and inhibited by PGI2 even at the maximum aggregation. Thrombi corresponding to either type of platelet aggregation can be produced in arterioles in the microcirculation. An ADP-induced thrombus, which is composed of Rev-Aggr of platelets and disaggregated automatically, is not inhibited by indomethacin, but inhibited by PGI2. In contrast, a stable thrombus, which is composed of Irrev-Aggr of platelets and keeps its initial shape for a longer period, is sensitive to indomethacin, but resistant to PGI2. Thus, anti-thrombotic drugs should be developed and used for targeting the individual processes of thrombus formation.