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V Ceña

Publications and source records attributed to V Ceña.

At least 37 records · Page 2Linked to original sources
PubMed1992

Chromostatin receptors control calcium channel activity in adrenal chromaffin cells.

One of the functions of chromogranin A (CGA), the major soluble component of secretory granules in both adrenal medullary chromaffin cells and many other endocrine cell types appears to be that of a prohormone. CGA is the precursor of several peptides including pancreastatin, a 49-residue peptide, and a 20-residue peptide, chromostatin, which have been identified as biologically active peptides. Chromostatin produces a dose-dependent inhibition (ID50 of 5 nM) of the secretagogue-evoked catecholamine secretion from chromaffin cells. Here we report that chromostatin potently inhibits L-type calcium currents recorded with the nystatin-perforated patch technique in cultured chromaffin cells. This inhibitory effect of chromostatin on calcium currents was not observed in experiments using the classical patch-clamp whole-cell approach which induces the leakage of cytoplasmic components. Using 125I-chromostatin, we show that chromostatin exhibits a fully reversible and saturable binding to the plasma membrane of cultured chromaffin cells. Analysis of binding experiments at equilibrium indicates the existence of one class of binding sites with a Bmax of 2.7 pmol/mg of chromaffin cell proteins and an apparent Kd of 6.5 nM. This high affinity is in good correlation with the half-maximal concentration (ID50 5 nM) of chromostatin inhibiting catecholamine secretion from chromaffin cells. Specificity of the chromostatin binding was further assessed by displacement experiments with unlabeled CGA-related or -unrelated peptides. We found an excellent quantitative correlation between the affinities of the various peptides determined by binding assays and their functional potency tested on catecholamine secretion: bovine chromostatin greater than human chromostatin greater than CGA much greater than rat chromostatin, pancreastatin, CAP-14, substance P, and Leu-enkephalin. Cross-linking experiments reveal that chromostatin associates specifically with an 80-kDa plasma membrane protein. These results together with the patch-clamp experiments support the idea that chromaffin cells possess specific chromostatin receptors and that activation of such receptors leads to the inhibition of L-type voltage-sensitive calcium channels through an intracellular second messenger pathway.

Adrenal Glands↗
PubMed1992

Axoplasmic transport of [3H]ouabain binding sites and catecholamine secretion from an adrenergic nerve trunk.

The presence of a functional Na+/Ca2+ exchange system was explored in the ligated cat hypogastric nerve, a preparation that has been proposed as a model of giant noradrenergic nerve terminal free of effector cells. The rationale for this study was to monitor noradrenaline secretion from the ligated cat hypogastric nerve promoted by the increase in intracellular Ca2+ levels after ouabain blockade of Na+,K(+)-ATPase molecules present in the plasma membrane of the ligated cat hypogastric nerve. Such an increase in intracellular Ca2+ levels is achieved by activation, in "reverse mode," of the Na+/Ca2+ exchange system. In the present study, [3H]ouabain binding sites were identified on crude preparations of hypogastric nerve membranes. A single, high affinity (Kd around 10 nM), binding site was observed in both ligated and nonligated nerves. The number of binding sites increased with the time of ligation, reaching a peak of about 1 pmol/mg of protein 48 hr after ligation. Blockade of these binding sites by ouabain induced a dose-dependent, Ca(2+)-dependent release of noradrenaline, with an ED50 around 50 microM. The maximum release amounted to 9% of the total noradrenaline content in the cells. As would be expected for ouabain-induced noradrenaline secretion mediated by a Na+/Ca2+ exchange system working in reverse mode, the effect of ouabain was dependent upon the presence of Na+ in the incubation medium, reaching a plateau at an extracellular Na+ concentration of 100 mM. Calcium uptake after Ca2+ reintroduction in ouabain-treated nerves increased with time of ligation, suggesting the incorporation of Na+/Ca2+ exchange carrier molecules into the axolemma of hypogastric nerves. The similarity between ouabain-induced noradrenaline secretion from the ligated cat hypogastric nerve and from other adrenergic systems strongly supports the idea that the ligated cat hypogastric nerve is equipped with a functional Na+/Ca2+ exchange system that would contribute to the regulation of intracellular Ca2+ levels. Furthermore, these data, together with previously published reports, fully characterize, from a biochemical point of view, the ligated hypogastric nerve as a model of giant noradrenergic nerve terminal free of effector cells.

Animals↗
PubMed1991

Pertussis toxin stimulation of catecholamine release from adrenal medullary chromaffin cells: mechanism may be by direct activation of L-type and G-type calcium channels.

We have previously shown that pertussis toxin (PTX) stimulates delayed-onset, [Ca2+]o-dependent catecholamine (CA) release from bovine chromaffin cells. We now show that this effect of PTX is inhibited in part (50%) by dihydropyridine Ca(2+)-channel antagonists niludipine and nifedipine, and is potentiated by the dihydropyridine Ca(2+)-channel agonist Bay K-8644. We and others have shown that pretreatment of chromaffin cells with PTX results in enhanced catecholamine secretion in response to high [K+]o, nicotine and muscarine, and here we extend these observations by showing that toxin pretreatment also enhances the secretory response to [Ba2+]o. All these data are consistent with the concept that PTX may act on Ca2+ channels. To examine the possibility of a direct action of the toxin on the voltage-gated L-type Ca2+ channel known to be present in these cells, we studied the effects of the toxin on whole cell Ca2+ currents. We found and report here that spontaneous electrical activity was considerably increased in PTX-treated cells. Our measurements of whole cell inward Ca2+ currents indicate that the underlying mechanism is a marked shift of the activation curve of the L-type Ca2+ current along the voltage axis towards more negative potentials. While treatment of the cells with PTX had no effect on L-type Ca(2+)-channel conductance (6 nS/cell at 2.6 mM [Ca2+]o). PTX evoked the activation of a new class of Ca(2+)-selective channels (5 pS in 25 mM [Ca2+]pipet), which are rather insensitive to membrane potential.(ABSTRACT TRUNCATED AT 250 WORDS)

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1991

Norepinephrine stimulates potassium efflux from pinealocytes: evidence for involvement of biochemical "AND" gate operated by calcium and adenosine 3',5'-monophosphate.

Biochemical studies of K+ efflux from rat pinealocytes revealed for the first time that norepinephrine (NE) increases 86Rb+ and 42K+ efflux. The effects of NE depend upon concurrent activation of both alpha 1- and beta-adrenoceptors. This effect is mediated by cAMP and Ca2+, which appear to act in conjunction to control K+ efflux; studies with charybdotoxin and tetraethylammonium indicate that a Ca2(+)-sensitive K+ channel (K(Ca] appears to be involved. Patch clamp studies identified a large conductance (approximately 100 psec) K+ channel. This study also revealed for the first time that NE treatment increases the fraction of time that this channel was open. Studies of inside-out pineal membrane patches indicated that increasing Ca2+ at the cytoplasmic surface of the membrane increased the frequency of channel opening, as is typical of K(Ca) channels in this type of preparation. Outward K+ currents were almost completely blocked by tetraethylammonium (10 mM) and scorpion venom (L. quinquestriatum; 100 ng/ml). Cell-attached studies confirm that the effects of NE are mediated by intracellular second messengers. These investigations suggest that NE elevates K+ flux, probably through a large conductance K(Ca) channel, that NE acts through alpha 1- and beta-adrenergic receptors, and that Ca2+ and cAMP act together through a biochemical "AND" gate to mediate the effects of receptor activation. Activation of this K(Ca) channel would have a hyperpolarizing influence and might contribute to the adrenergic hyperpolarization of pinealocytes.

Animals↗
PubMed1990

Kinetic characteristics of calcium-dependent, cholinergic receptor controlled ATP secretion from adrenal medullary chromaffin cells.

Adrenal chromaffin cells secrete catecholamines (CA) and ATP in response to acetylcholine (ACh) and high [K+]o. The release process is relatively fast making it difficult to measure the early phase of the secretory response. Recently we were able to resolve the time course of the secretory response by measuring the release of ATP using luciferin-luciferase included in the extracellular medium. For the three secretagogues studied, ACh, nicotine and high [K+]o, the early phase of release followed a complex kinetics. Allowing for an initial delay of the secretory response, the kinetics could be described as the sum of two power exponential processes. Increasing the temperature from 23 to 37 degrees C induced a marked decrease in the two time constants needed to fit the early time course of the ATP secretion. The activation energies, estimated from Arrhenius plots, were approx. 20 and 16 kcal/mol for both phases of ATP release induced by either cholinergic agonists or high [K+]o. These results suggest that cholinergic receptor activation and membrane depolarization induce ATP (and CA) secretion through a common pathway. The initial delay in the onset of the secretory response decreased with increasing doses of secretagogue and with temperature. We propose that the delay preceding the actual onset of ATP release represents the time required for generation of intracellular second messengers. The effective concentration attained by these messengers depend apparently on both receptor occupancy by the agonist and the ensuing Ca2+ channel activation.

Acetylcholine↗
PubMed1990

Differential expression and enzymatic properties of the Na+,K(+)-ATPase alpha 3 isoenzyme in rat pineal glands.

We have used immunoblotting and biochemical techniques to analyze expression of Na+,K(+)-ATPase alpha and beta subunits in rat pineal glands. Western blot analysis of pineal microsomal membrane fractions with antisera specific for each of the three rat alpha and two rat beta subunits revealed similar levels of expression of alpha 1 and alpha 3 subunits in pineal glands of 5-day-old rats. High levels of alpha 3 and beta 2 subunits and low levels of alpha 1 subunits were detected in adult glands. No alpha 2 or beta 1 subunits were detectable at either developmental stage. Examination of the enzymatic properties of the pineal gland alpha 3 isoform suggests that this enzyme is a ouabain-sensitive ATPase whose activity is dependent upon Na+ and K+. This ATPase exhibited a lower apparent Km for Na+ than the kidney alpha 1 isoenzyme and did not show positive cooperative Na+ activation. Our results suggest that the activity of the Na+,K(+)-ATPase alpha 3 isoenzyme may be adapted to function under conditions of hyperpolarizing transmembrane potentials.

Aging↗
PubMed1989

Effects of calcium and Bay K-8644 on calcium currents in adrenal medullary chromaffin cells.

The kinetic and steady-state characteristics of calcium currents in cultured bovine adrenal chromaffin cells were analyzed by the patch-clamp technique. Whole cell inward Ca2+ currents, recorded in the presence of either 5.2 or 2.6 mM Ca2+ exhibited a single, noninactivating component. To analyze the effects of Ca2+ and Bay K-8644 on the kinetics of the Ca2+ currents, we used a modified version of the Hodgkin-Huxley empirical model. At physiological [Ca2+] (2.5 mM) the midpoint of the steady-state Ca2(+)-channel activation curve lay at -6.9 mV. Increasing the [Ca2+] to 5.2 mM shifted the midpoint by -4.3 mV along the voltage axis. At the midpoint, changes in potential of 7.8 mV (for 5.2 mM Ca2+) and 9.2 mV (for 2.5 mM Ca2+) induced an e-fold change in the activation of the current. Increasing [Ca2+]o from 2.5 to 5.2 mM induced a marked increase in the rate constant for turning on the Ca2+ permeability. Conductances were estimated from the slope of the linear part of the current-voltage relationships as 8.7 and 4.2 nS in the presence of 5.2 and 2.5 mM Ca2+, respectively. Incubation of the cells in the presence of Bay K-8644 at increasing concentrations from 0.001 to 0.1 microM increased the slope conductance from 4.2 to 9.6 nS. Further increases in the concentration of Bay K-8644 from 1 to 100 microM induced a marked reduction in the conductance to 1.1 nS. In the presence of Bay K-8644 (0.1 microM) the midpoint of the activation curve was shifted by 6.1 mV towards more negative potentials, i.e., from -6.9 to -13 mV. At the midpoint potential of -13 mV, a change in potential of 6.9 mV caused an e-fold change in Ca2+ permeability. The kinetic analysis showed that Bay K-8644 significantly reduced the size of the rate constant for turning off the Ca2+ permeability.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1988

Ouabain induces acetylcholine release from pure cholinergic synaptosomes independently of extracellular calcium concentration.

We have studied the correlation between [3H]ouabain binding sites, (Na+ + K+)ATPase (EC 3.6.1.3) activity and acetylcholine (ACh) release in different subcellular fractions of Torpedo marmorata electric organ (homogenate, synaptosomes, presynaptic plasma membranes). Presynaptic plasma membranes contained the greater number of [3H]ouabain binding sites, in good agreement with the high (Na+ + K+)ATPase activity found in this fraction. Blockade of this enzymatic activity by ouabain dose-dependently induced ACh release from pure cholinergic synaptosomes, either in the presence or absence of extracellular calcium ions. We suggest that one of the mechanisms involved in the ouabain-induced ACh release in the absence of Ca2+o may be an increase in Na+i that could (a) evoke Ca2+ release from internal stores and (b) inhibit ATP-dependent Ca2+ uptake by synaptic vesicles.

Acetylcholine↗
PubMed1987

Developmental study of ouabain inhibition of adrenergic induction of rat pineal serotonin N-acetyltransferase (EC 2.3.1.87).

The activity of arylalkylamine N-acetyltransferase (EC 2.3.1.87), the rate-controlling enzyme in melatonin synthesis is stimulated approximately equal to 100-fold by an adrenergic cyclic AMP mechanism in both neonatal and adult rat pineal glands. This stimulation is blocked in the adult gland by the depolarizing agents ouabain (1 microM) and K+ (80 mM) (Parfitt, A., Weller, J.L., Klein, D.C., Sakai, K.K., and Marks, B.H. (1975) Mol. Pharmacol. 11, 241-255). In the present study pineal glands obtained from prenatal to adult rats were used; it was found that K+ (80 microM) inhibited the adrenergic stimulation of N-acetyltransferase activity at all ages but that ouabain (1 nM to 1 mM) treatment was not inhibitory early in development. In contrast, in the neonate, ouabain (1-100 nM) enhanced adrenergic induction of N-acetyltransferase activity, and ouabain treatment alone (1-1000 nM) stimulated N-acetyltransferase activity. A small stimulation was also seen at one concentration (1 nM) in the adult. Analysis of the development of high affinity ouabain binding sites and Na+,K+-ATPase activity in the intact pineal gland indicated that the developmental pattern of both resemble the development of ouabain inhibition of the adrenergic stimulation of N-acetyltransferase activity. All are low for the first few days of life, gradually increase during the next 3 weeks of life, and then approach adult levels. Similarly, ouabain (1 nM to 1 mM) had no effect on 86Rb uptake in the 2-day-old gland but blocked (IC50 congruent to 20 nM) 86Rb uptake in the adult gland. These findings indicate ouabain probably has little inhibitory effect on the norepinephrine stimulation of N-acetyltransferase activity in the neonatal because a high affinity ouabain binding form of Na+,K+-ATPase activity, similar to the alpha + form identified in rat brain, is at very low levels in the pinealocyte. Accordingly, it appears that an ouabain-insensitive mechanism in the neonatal gland maintains membrane potential and that this mechanism plays a less important role in the adult. The explanation of why ouabain alone stimulates N-acetyltransferase activity and why it enhances the effects of norepinephrine in the neonatal pineal gland might be that ouabain acts on surviving neural elements present in the gland to cause the net release of a transmitter, perhaps norepinephrine, which then stimulates N-acetyltransferase activity.

Acetyltransferases↗
PubMed1987

Cardiac glycosides stimulate phospholipase C activity in rat pinealocytes.

Ouabain and related cardiac glycosides stimulate phospholipase C activity 5-fold in rat pinealocytes. The combined treatment of ouabain and norepinephrine, which also stimulates phospholipase C, produces an additive effect. The effects of either ouabain or norepinephrine are blocked by EGTA. However, there are notable differences. The stimulatory effect of ouabain is lost when extracellular Na+ is reduced to 20 mM and is not blocked by prazosin. In contrast, the stimulatory effect of norepinephrine is not blocked when extracellular Na+ is reduced to 20 mM but is blocked by prazosin. Ouabain appears to increase phospholipase C activity through a mechanism involving inhibition of Na+,K+-ATPase, and an accumulation of intracellular Na+ and Ca2+, not involving alpha 1-adrenoceptors. These findings raise the possibility that activation of phospholipase C might be a more general effect of cardiac glycosides.

Animals↗
PubMed1987

Inhibition of adrenomedullary catecholamine release by propranolol isomers and clonidine involving mechanisms unrelated to adrenoceptors.

1 Transmural electrical stimulation (10 Hz, 40 V, 1 ms for 60s) increased total catecholamine secretion from perfused cat adrenal glands; this response was enhanced by neostigmine and inhibited by mecamylamine, suggesting that release of acetylcholine from splanchnic nerve terminals was stimulating nicotinic receptors and enhancing catecholamine secretion. 2 Isoprenaline, (-)-propranolol and (+)-propranolol (10(-7)-10(-5)M) inhibited the electrically-evoked secretory response by 40-70%; similar reductions were obtained with clonidine and yohimbine. Neither, (+)-propranolol nor (-)-propranolol inhibited K-evoked secretion from cat adrenals; in contrast, nimodipine potently inhibited it (IC50 = 24 nM). 3 Either, racemic propranolol or the (+)- or (-)-isomers (1-10 microM) equally inhibited [3H]-noradrenaline release evoked by nicotine or acetylcholine from cultured bovine adrenal chromaffin cells; clonidine (10 microM) inhibited secretion by 50% and yohimbine or isoprenaline did not affect it. 4 The results indicate that adrenomedullary catecholamine release evoked by splanchnic nerve stimulation is not modulated by alpha- or beta-adrenoceptors and suggest that propranolol may inhibit secretion by blocking ion fluxes through the acetylcholine receptor ionophore. Clonidine may inhibit secretion by this same mechanism, and/or by interfering with some intracellular event in the secretory mechanism.

Adrenal Medulla↗
PubMed1987

Characterization of the alpha +-like Na+,K+-ATPase which mediates ouabain inhibition of adrenergic induction of N-acetyltransferase (EC 2.3.1.87) activity: studies with isolated pinealocytes.

Ouabain inhibits (IC50 congruent to 200 nM) the congruent to 100-fold adrenergic cyclic AMP stimulation of rat pineal arylalkylamine N-acetyltransferase (EC 2.3.1.87, serotonin N-acetyltransferase, NAT) activity in intact pineal glands. In the present study, ouabain binding sites in pineal membranes were characterized in detail and compared to sites in isolated pinealocytes, which mediate the inhibition of Na+,K+-ATPase, as indicated by 86Rb uptake and norepinephrine (NE) stimulation of NAT activity. High affinity ouabain-binding sites were identified in crude preparations of pineal membranes (Kd congruent to 14 nM; Bmax congruent to 4 pmol/mg of protein) and similar sites were also found in ovine and bovine pineal tissue. The ouabain Kd value for the rat pineal binding sites was similar to the estimated ouabain IC50 values for 86Rb uptake and the NE stimulation of NAT activity in intact rat pinealocytes. In addition, the relative orders of potency of four cardiac glycosides in displacing [3H]ouabain from high affinity binding sites and inhibiting both 86Rb uptake and NE stimulation of NAT activity were the same (acetyldigitoxin greater than ouabain greater than digitoxin greater than strophanthidin). The similarities in the characteristics of the high affinity [3H]ouabain-binding sites and the sites involved in the inhibition of 86Rb uptake and stimulation of NAT activity indicate that an alpha +-like Na+,K+-ATPase mediates the inhibitory effects of ouabain on the adrenergic induction of pineal NAT activity.

Acetyltransferases↗
PubMed1986

Presence of a noradrenaline uptake system on a ligated cat sympathetic nerve.

[3H]noradrenaline [( 3H]NA) uptake studies were carried out in cat hypogastric nerves ligated in vivo, 2 cm distal to the inferior mesenteric ganglion, for different time periods. Atria from the same animals served as controls to determine the uptake of the amine by the U1 uptake system present in noradrenergic nerve terminals. The net uptake of [3H]NA by hypogastric nerves increased with time of ligation, reaching a maximum 24 h after ligation in the segment of nerve immediately proximal to the ligature (P1 segment, neurosome). No further increase in uptake was observed at 48 or 72 h. Segments distal (D1) to the ligature also retained significant amounts of [3H]NA. In both cases the uptake was blocked by cocaine (3 microM). Reserpine pre-treatment (2 mg/kg I.M.) markedly decreased the endogenous NA content to 1-2% of untreated cats and the net uptake of [3H]NA was lowered to 25% both in cat hypogastric nerve and atria. The uptake was further decreased in the presence of cocaine (3 microM). 6-Hydroxydopamine (100 microM) did not modify the [3H]NA uptake by ligated cat hypogastric nerves but almost abolished the [3H]NA net uptake by right atria from the same animals. After collagenase pre-treatment (0.05% for 15 min) the net uptake of [3H]NA was not altered in the atrium. However, collagenase-pre-treated ligated nerves, took up almost twice as much [3H]NA; under these conditions, 6-hydroxydopamine produced a marked decrease in [3H]NA net uptake. These data suggest the presence in the perineurium of a diffusion barrier for very polar substances, including 6-hydroxydopamine. In conclusion, our data demonstrate that the cat hypogastric nerve ligated in vivo has a cocaine-sensitive system for NA uptake which resembles the NA uptake mechanism (U1) present in noradrenergic nerve terminals. Our data further support the view that the ligated cat hypogastric nerve (neurosome) could be considered as a model of noradrenergic nerve terminal free of effector cell.

Animals↗
PubMed1986

Retinal S-antigen: immunocytochemical and immunochemical studies on distribution in animal photoreceptors and pineal organs.

Antiserum against bovine retinal S-antigen, a soluble protein (MW = 50 kDa) thought to be involved in phototransduction, was used in an immunohistochemical and immunochemical study of vertebrate eyes and pineal systems and invertebrate photoreceptor organs. Positive reactions, not seen with antiserum preabsorbed with highly purified S-antigen, were observed in planarian and starfish ocelli; scallop eyes; polychaete eye; crayfish compound eye; lamprey, salmon, frog, turtle, quail and hamster eyes. A specific reaction was also seen in the pineal organ of all the vertebrates examined, albeit weak in turtle and quail. In addition, several structures associated with photoreceptor organs, including the reduced frontal eyes of crayfish, the organ of Bellonci in crayfish eyestalk, and bipolar cells resembling those giving rise to Landolt's clubs in quail and golden hamster retinae, were immunopositive. Immunochemical studies revealed the presence of a single immunopositive band of protein which was similar but not identical in size in all vertebrate eyes and pineal organs (except that of chicken pineal) and invertebrate tissue examined. The wide distribution of positive reaction in photoreceptive tissue indicates that the retinal S-antigen determinant has been highly conserved during evolution.

Animals↗
PubMed1985

Effect of the dihydropyridine Bay K 8644 on the release of [3H]-noradrenaline from the rat isolated vas deferens.

The effects of Bay K 8644 on the release of [3H]-noradrenaline evoked by potassium, electrical stimulation or tyramine from the rat isolated vas deferens labelled with [3H]-noradrenaline were investigated. Bay K 8644 (1 microM) by itself did not affect the spontaneous release of tritium from the rat isolated vas deferens. However, it increased the calcium-dependent release of tritium elicited by both high potassium (59 mM) and electrical field stimulation. The exposure of rat vas deferens to phentolamine (10 microM) increased the release of tritium induced by potassium (59 mM) and electrical field stimulation. Bay K 8644 (1 microM) failed to increase further the release of tritium elicited by both stimuli in preparations previously treated with phentolamine (10 microM). The calcium-independent release of [3H]-noradrenaline evoked by tyramine (10 microM) was not affected by Bay K 8644 (1 microM). The results of our study support the view that alpha2-adrenoceptors modulate noradrenaline release by restricting calcium influx into sympathetic nerve terminals through voltage-dependent channels.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1985

Ion dependence of the release of noradrenaline by tetraethylammonium and 4-aminopyridine from cat splenic slices.

Cat splenic slices prelabelled with [3H]-noradrenaline were incubated in oxygenated Krebs-bicarbonate solution at 37 degrees C, and the spontaneous total 3H release into different incubation media monitored. In normal Krebs bicarbonate solution, the spontaneous tritium fractional release amounted to 3.7% of the tissue radioactivity content per 5 min collection period. Tetraethylammonium (TEA) increased spontaneous transmitter release in a concentration-dependent manner; the release was maximal at 30 mM and was 3.5 times the basal release. 4-Aminopyridine (4-AP) also enhanced the spontaneous release of tritium. The response increased linearly with 4-AP concentration (1-10 mM). With 10 mM 4-AP, the release was as much as 6 times the basal transmitter release. Guanidine was much less potent than either TEA or 4-AP. The secretory response to TEA or 4-AP was little affected by changes in external Ca2+, Mg2+, Na+, Cl-, H2PO4- or by tetrodotoxin. However, transmitter release evoked by TEA or 4-AP strongly depended upon the concentration of HCO3- of the incubation solution; in fact, the secretory response varied almost linearly between 1 and 25 mM HCO3-. The mechanisms underlying these effects are probably related to the well-known ability of TEA and 4-AP to block K+ conductance that would cause depolarization of the splenic sympathetic nerve terminals. The HCO3- requirements for the secretory response are probably related to the ability of CO2/HCO3- solutions to mobilize and release Ca2+ from intracellular organelles.

4-Aminopyridine↗