Inhibitory effect of 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) in vascular smooth muscle

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.

Cyclosporin effect on rat aorta alpha(1)-adrenoceptors and their transduction mechanisms

A possible explanation for cyclosporin-induced arterial hypertension may be its action on the adrenergic system. In spite of the controversial results reported in literature, it seems that cyclosporin changes the vascular response to noradrenaline. Therefore, after observation that two cyclosporin doses increase rat blood pressure and vascular reactivity in response to noradrenaline, the aim of this work was to study the cellular mechanisms beside the cyclosporin-induced changes in response to noradrenaline. Therefore, the cyclosporin influence on alpha(1)-adrenoceptors as well as on their transduction mechanism in smooth muscle cells was studied. Through Scatchard analysis of specific [(3)H]-prazosin binding, the alpha(1)-adrenoceptor number and related affinity were studied, before and after cyclosporin exposure. The cyclosporin influence on alpha1-adrenoceptor transduction mechanisms was also evaluated by the quantification of intracellular free calcium contents [Ca2+]i and inositol phosphate (InsP) turnover. All in vitro experiments were performed in rat aortic smooth muscle cells in culture. Results showed that both cyclosporin concentrations (10(-6) and 10(-7) M) changed alpha1-adrenoceptor number but only 10(-7) M cyclosporin increased its affinity for [(3)H]-prazosin. Compared with control cells, only 10(-7) M cyclosporin increased InsP levels. Stimulation by noradrenaline increased InsP in 10(-7) M cyclosporin-treated cells but decreased InsP in the presence of 10(-6) M cyclosporin. Both cyclosporin concentrations increased [Ca2+]i in basal conditions and after noradrenaline stimulation. The results suggest that after noradrenaline stimulation cyclosporin increases [Ca2+]i, probably through different mechanisms, depending on the cyclosporin concentration used. However, 10(-7) M cyclosporin increases alpha1-adrenoceptor affinity and their related transduction mechanisms. The higher cyclosporin concentration (10(-6) M) seems to induce downregulation of alpha1-adrenoceptors, probably by activation of protein kinase C.

Mechanically induced calcium waves in articular chondrocytes are inhibited by gadolinium and amiloride

Chondrocytes in articular cartilage utilize mechanical signals from their environment to regulate their metabolic activity. However, the sequence of events involved in the transduction of mechanical signals to a biochemical signal is not fully understood. It has been proposed that an increase in the concentration of intracellular calcium ion ([Ca2+]i) is one of the earliest events in the process of cellular mechanical signal transduction. With use of fluorescent confocal microscopy, [Ca2+]i was monitored in isolated articular chondrocytes subjected to controlled deformation with the edge of a glass micropipette. Mechanical stimulation resulted in an immediate and transient increase in [Ca2+]i. The initiation of Ca2+ waves was abolished by removing Ca2+ from the extracellular media and was significantly inhibited by the presence of gadolinium ion (10 microM) or amiloride (1 mM), which have previously been reported to block mechanosensitive ion channels. Inhibitors of intracellular Ca2+ release (dantrolene and 8-diethylaminooctyl 3,4,5-trimethoxybenzoate hydrochloride) or cytoskeletal disrupting agents (cytochalasin D and colchicine) had no significant effect on the characteristics of the Ca2+ waves. These findings suggest that a possible mechanism of Ca2+ mobilization in this case is a self-reinforcing influx of Ca2+ from the extracellular media, initiated by a Ca2+-permeable mechanosensitive ion channel. Our results indicate that a transient increase in intracellular Ca2+ concentration may be one of the earliest events involved in the response of chondrocytes to mechanical stress and support the hypothesis that deformation-induced Ca2+ waves are initiated through mechanosensitive ion channels.

Calcium recruitment in renal vasculature: NE effects on blood flow and cytosolic calcium concentration

This study provides new information about the relative importance of Ca2+ mobilization and entry in the renal vascular response to adrenoceptor activation. We measured renal blood flow (RBF) in Sprague-Dawley rats in vivo using electromagnetic flowmetry. We measured intracellular free Ca2+ concentration ([Ca2+]i) in isolated afferent arterioles utilizing ratiometric photometry of fura-2 fluorescence. Renal arterial injection of NE produced a transient decrease in RBF. The response was attenuated, in a dose-dependent manner, up to approximately 50% by nifedipine, an antagonist of L-type Ca2+ entry channels. Inhibition of Ca2+ mobilization by 3,4, 5-trimethoxybenzoic acid-8-(diethylamino)octyl ester (TMB-8) inhibited the renal vascular effects of NE in a dose-dependent manner, with maximal blockade of approximately 80%. No additional attenuation was observed when nifedipine and TMB-8 were administered together. In microdissected afferent arterioles, norepinephrine (NE; 10(-6) M) elicited an immediate square-shaped increase in [Ca2+]i, from 110 to 240 nM. This in vitro response was blocked by nifedipine (10(-6) M) and TMB-8 (10(-5) M) to a degree similar to that of the in vivo experiments. A nominally calcium-free solution blocked 80-90% of the [Ca2+]i response to NE. The increased [Ca2+]i elicited by depolarization with medium containing 50 mM KCl was totally blocked by nifedipine. In contrast, TMB-8 had no effect. Our results indicate that both Ca2+ entry and mobilization play important roles in the renal vascular Ca2+ and contractile response to adrenoceptor activation. The entry and mobilization mechanisms activated by NE may interact. That a calcium-free solution caused a larger inhibition of the NE effects on afferent arterioles than nifedipine suggests more than one Ca2+ entry pathway.

Roles of outward potassium currents in the action potentials of guinea pig ureteral myocytes

Outward currents of freshly dissociated ureteral myocytes consist mainly of Ca(2+)-activated K+ current (IKCa) and a transient outward current (ITO). No delayed rectifier current was apparent. IKCa is small and nondecaying and fluctuates actively and irregularly. Blocking IKCa decreased resting membrane conductance and prolonged action potential plateaus, showing its roles in maintaining the resting potential and in repolarizing action potentials. It is also responsible for the membrane potential fluctuations on action potential plateaus. Neither 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride nor caffeine reduced the fluctuations in the outward current or in the action potentials, indicating that internal Ca2+ storage contributes little to the fluctuations. ITO has fast activation and inactivation kinetics with inactivation time constants of approximately 15 and 150 ms, respectively. Its highly negative voltage-availability relationship (V0.5 = -70.5 mV) suggests a low availability (< 5%) at normal resting potentials. It has only trivial effects on action potentials.

Activation by high potassium of a novel voltage-operated Ca2+ channel in rat spleen

1. High potassium produced a concentration-dependent contraction in rat isolated spleen. 2. The high potassium-induced contraction of rat spleen was abolished in Ca(2+)-free Krebs solution containing 1 mM EGTA, and the subsequent addition of 3 mM Ca2+ restored the high potassium-induced contraction to the control level. 3. Nifedipine, verapamil, diltiazem, Cd2+, Ni2+, Co2+, R-(+)-Bay K 8644 and pimozide inhibited and relaxed high potassium-induced contraction of rat spleen with IC50 and EC50 values much higher than those values in rat aorta. 4. In addition, high potassium-stimulated contraction of rat spleen was insensitive to omega-conotoxin GVIA, omega-conotoxin MVIIC and omega-agatoxin IVA. 5. The high potassium-induced contraction of rat spleen was also unaffected by tetrodotoxin (TTX), prazosin, chloroethylclonidine (CEC), yohimbine, propranolol, atropine, diphenhydramine, cimetidine, ketanserin, 3-tropanyl-indole-3-carboxylate, saralasin, indomethacin, nordihydroguaiaretic acid, GR32191B, domperidone, naloxone, chlorpromazine, suramin, (+/-)-2-amino-5-phosphonopentanoic acid, 6,7-dinitroquinoxaline-2,3-dione (DNQX), L-659,877, L-703,606, lorglumide, PD 135,158 N-methyl-D-glucamine, benextramine, amiloride, dantrolene, TMB-8, econazole, staurosporine and neomycin. 6. Forskolin and sodium nitroprusside relaxed high potassium-induced contraction of rat spleen with EC50 values of 0.55 +/- 0.04 and 20.0 +/- 2.7 microM, respectively. 7. It is concluded that high potassium may activate a novel, pharmacologically uncharacterized voltage-operated Ca2+ channel in rat spleen.

Ca2+ mobilization by caffeine in single smooth muscle cells of the rat tail artery

The fluorescent dye fura-2 was used to study the effects of caffeine on cytosolic free Ca2+ level ([Ca2+]i) in freshly isolated single cells from the rat tail artery. Caffeine caused a concentration-dependent transient increase in [Ca2+]i and shortening of the cell. At higher concentrations (> 2 mM), a tonic increase in [Ca2+]i was also observed. The caffeine-induced changes in [Ca2+]i were reproducible with repeated challenges, even though the cells had contracted due to previous exposure to caffeine. Removal of extracellular Ca2+ reduced the resting [Ca2+]i to about half and abolished the tonic Ca2+ increase to caffeine. The transient component was not significantly affected to the first caffeine challenge after Ca2+ removal, but was abolished to the second challenge. Ryanodine (10 microM) significantly inhibited the responses to caffeine while nifedipine and TMB-8-(8-(diethylamino)octyl ester of 3,4,5-trimethoxybenzoic acid) were not effective. Thapsigargin (10-100 microM) induced a sustained increase in [Ca2+]i to 67 nM. The response of caffeine was not affected by thapsigargin. Pretreatment of the cells with noradrenaline (10 microM) abolished subsequent response to caffeine. These results show that Ca2+ responses to caffeine in single cells from the rat tail artery are reproducible with repeated caffeine challenge. Therefore, single cells can be used for comparison studies of the effects of pharmacological agents.

Enhancement of cell killing and increase in cytosolic calcium concentration by combined treatments with hyperthermia and TMB-8 in mouse mammary carcinoma FM3A cells

Mouse mammary carcinoma FM3A cells were exposed 42 degrees C or 44 degrees C hyperthermia for 60 min in a culture medium with or without 8-(N,N-diethylamino) octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8) at a concentration of 100 microM. Cell survival, cytosolic calcium ion concentration, [Ca2+]C, and influx of 45Ca2+ were investigated to examine the cardinal role of intracellular Ca2+ in hyperthermic cell killing. The [Ca2+]C in a single cell was measured by a digital image analyzing technique using Fura-2. TMB-8 enhanced cell killing, increase in [Ca2+]C, and influx of 45Ca2+ induced by both hyperthermic treatments. In contrast, TMB-8 abolished the increase in [Ca2+]C induced by hyperthermia at 42 degrees C and 44 degrees C in the absence of extracellular Ca2+. These results suggest that TMB-8, an agent usually called an inhibitor of intracellular calcium mobilization at 37 degrees C or lower, enhances Ca2+ entry from the extracellular space at 42 degrees C or 44 degrees C and, in turn, promotes cell killing under hyperthermic conditions.

Effect of TMB-8 on alpha-adrenoceptor agonist and KC1 induced-contractions in isolated rabbit aorta

1. TMB-8 (10(-6) M-10(-4) M) depresses the contractile effect of the selective alpha 1-adrenoceptor agonists methoxamine and phenylephrine in the isolated rabbit aorta. 2. TMB-8 also depresses contractions evoked by 80 mM KCl in this tissue when used at similar concentrations. 3. The calcium antagonist nifedipine potentiates the inhibitory effect of TMB-8 on the alpha 1-contractions. 4. In preparations mounted in Ca-free solution containing 0.5 mM EGTA, 10(-4) M TMB-8 markedly depressed the contractions caused by both alpha 1-adrenoceptor agonists. 5. The Ca2+ agonist BAY K 8644 (10(-6) M) partially prevented the inhibitory effect of TMB-8 on 80 mM KCl contractions.

Effect of Ca2+ modulators on acetylcholine-induced phasic and tonic contractions and A23187-induced contractions in ileal longitudinal muscle and IP3 production

The influence of different sources of Ca2+ on the Emax and ED50 values of acetylcholine (ACh)-induced phasic and tonic contractions and on A23187-induced contractions was studied using different extracellular Ca2+ concentrations ([Ca2+]o) and the Ca2+ modulators TMB-8 and D600. IP3 production induced by both stimulants was also studied. The results are compatible with: (a) the mobilization of Ca2+ from an intracellular source as a primary event in the phasic response. (b) The primary involvement of a D600-sensitive inward Ca2+ current in the ACh-tonic response. (c) An inward D600-sensitive Ca2+ current associated with the ionophore transported ion. (d) The involvement of an IP3 independent, TMB-8 sensitive mechanism of Ca2+ mobilization involved in the A23187-induced responses.

TMB-8 (8-(N,N-diethylamino) octyl-3,4,5-trimethoxybenzoate) inhibits the ATP-sensitive K+ channel

Whole-cell current clamp, single-channel recordings and 86Rb+ flux techniques have been used to show that 8-(N,N-diethyl-amino)octyl-3,4,5-trimethoxybenzoate (TMB-8) inhibits ATP-sensitive K+ channels in HIT-T15 beta-cells. TMB-8 inhibition is observed when KATP channels are activated by ATP depletion or by the K+ channel opener, diazoxide.