Spatial view cells in the primate hippocampus and memory recall.

Hippocampal spatial view neurons in primates provide allocentric representations of a view of space 'out there'. The responses depend on where the monkey is looking; and can be updated by idiothetic (self-motion) inputs provided by eye movements when the view is hidden. In a room-based object-place memory task, some hippocampal neurons respond to the objects shown, some to the places viewed, and some to combinations of the places viewed and the objects present in those locations. In an object-place recall task when the location in space at which an object has been seen is recalled by the presentation of the object, some primate hippocampal neurons maintain their responding to the object recall cue in a delay period without the object visible while the place is being recalled; and other neurons respond to the place being recalled. Other spatial view neurons form associations with the rewards present at particular locations in space. These findings, and computational models of the hippocampus, help to show how the primate including human hippocampus is involved in episodic memory.

Recognition memory: neuronal substrates of the judgement of prior occurrence.

Recognition memory relies on two processes: (i) identification and (ii) judgement concerning prior occurrence. A system centred on perirhinal cortex appears to be responsible for judgement of prior occurrence based on discrimination of the familiarity of stimuli or their recency of occurrence; in contrast, a hippocampal system probably supplies information concerning the episodic, contextual aspects of recognition memory. This review chiefly concerns the perirhinal system and, in particular, neurones that signal the prior occurrence of stimuli by a decrease in response. Details concerning such decremental responses are given and it is argued that such responses in perirhinal cortex are adequate for and central to discrimination of stimulus familiarity and recency in a wide range of situations. Information is given of similar types of neuronal responses in anatomically related brain regions and what may be deduced about the operation of the recognition memory system. The possibility is discussed that the neuronal responses that signal information concerning the recent occurrence of stimuli may contribute to repetition priming as well as recognition memory. Other described changes in the activity of individual neurones such as response enhancements, or sustained (delay) activity may allow solution of specialised forms of recognition memory tasks where relatively short-term working memory is adequate. Implications of the multi-faceted nature of recognition memory for the interpretation of results are emphasised. Unsolved problems and avenues for future experimentation, including determining the nature of possible underlying synaptic plastic changes, are discussed.

Effects of inorganic phosphate and ADP on calcium handling by the sarcoplasmic reticulum in rat skinned cardiac muscles.

OBJECTIVE: The aim was to investigate whether, and how, increases in inorganic phosphate (Pi) and ADP, similar to those occurring intracellularly during early myocardial ischaemia, affect the calcium handling of the sarcoplasmic reticulum. METHODS: Rat ventricular trabeculae were permeabilised with saponin. The physiological process of calcium induced calcium release (CICR) from the muscle sarcoplasmic reticulum was triggered via flash photolysis of the "caged Ca2+", nitr-5. Alternatively, calcium release was induced by rapid application of caffeine to give an estimate of sarcoplasmic reticular calcium loading. The initial rate of sarcoplasmic reticular calcium pumping was also assessed by photolysis of caged ATP at saturating [Ca2+]. Myoplasmic [Ca2+] (using fluo-3) and isometric force were measured. RESULTS: Pi (2-20 mM) significantly depressed the magnitude of CICR and the associated force transient. Sarcoplasmic reticular calcium loading was inhibited even more than CICR by Pi, suggesting that reduced calcium loading could account for all of the inhibitory effect of Pi on CICR and that Pi may slightly activate the calcium release mechanism. The reduced sarcoplasmic reticular calcium loading seemed to be due to a fall in the free energy of ATP hydrolysis (delta GATP) available for the calcium pump, since equal decreases in delta GATP produced by adding both Pi and ADP in various ratios caused similar falls in the calcium loading of the sarcoplasmic reticulum. The caged ATP experiments indicated that Pi (20 mM) did not affect the rate constant of sarcoplasmic reticular calcium uptake. ADP (10 mM) alone, or with 1 mM Pi, inhibited calcium loading. In spite of this, ADP (10 mM) did not alter CICR and, when 1 mM Pi was added, ADP increased CICR above control. CONCLUSIONS: An increase in intracellular Pi reduces sarcoplasmic reticular calcium loading and thus depresses the CICR. This could be an important contributing factor in the hypoxic or ischaemic contractile failure of the myocardium. However the detrimental effect of Pi may be offset to some extent by a stimulatory action of ADP on the calcium release mechanism of CICR.

Ca(2+)- and caffeine-induced Ca2+ release from the sarcoplasmic reticulum in rat skinned trabeculae: effects of pH and Pi.

OBJECTIVE: Our aims were: (1) to examine the effect of pH (7.4-6.5) on Ca2+ release from the sarcoplasmic reticulum (SR) of cardiac muscle, and (2) to see if these effects were altered by phosphate (Pi). METHODS: Rat ventricular trabeculae were permeabilised with saponin. Ca(2+)-induced Ca2+ release (CICR) from the SR was triggered by flash photolysis of nitr-5. Under similar loading conditions, SR Ca2+ loading was assessed using caffeine to release the Ca2+ in the SR. Force and fluo-3 fluorescence (a measure of the cytosolic [Ca2+]) were monitored. RESULTS: SR Ca2+ loading was optimal at pH 7.1 and was significantly reduced at pH 7.4, 6.8 and 6.5. CICR was the same at pH 7.4 as at pH 7.1, but was reduced, by more than Ca2+ loading, in acidic solutions. These differential effects on loading and CICR suggested that Ca2+ activation of the Ca2+ release channel was decreased (by > 50%) as pH was lowered from 7.4 to 6.5. A direct effect on the Ca2+ release channel was confirmed by the finding that Ca2+ release was slower in acidic solutions. Acidosis also slowed the re-uptake of Ca2+ into the SR after CICR, which may account for the reduced Ca2+ loading at low pH. As observed previously, Pi (20 mM) by itself decreased SR Ca2+ loading. However, the inhibitory effects of acidosis and Pi on SR Ca2+ loading were independent. CONCLUSIONS: A fall of pH over the range 7.4-6.5 directly inhibits the SR Ca2+ release channel. In addition, acidosis inhibits SR Ca2+ accumulation by a mechanism independent of that of Pi. Both effects of acidosis would act to decrease SR Ca2+ release and so would contribute to the negative inotropic actions of intracellular acidosis in intact cardiac muscle.

Differential neuronal encoding of novelty, familiarity and recency in regions of the anterior temporal lobe.

Activity of 2072 neurones was recorded in the anterior temporal lobe--in area TE, perirhinal cortex, entorhinal cortex and hippocampus--during performance of a visual recognition task by monkeys. In area TE, perirhinal cortex and entorhinal cortex, 454 neurones (38% of the 1162 visually responsive neurones) responded differentially on the basis of the relative familiarity or recency of presentation of the stimuli; in the hippocampus only one (3%) of its 40 visually responsive neurones) did so. The differentially responsive neurones were classified into those signalling information concerning the recency (19%), familiarity (37%) or novelty (38%) of stimuli. For 98% of these neurones a decreased response signalled that stimuli had occurred previously: no large response increments were observed. The mean differential latency of each of these types of neurone was shorter (approximately 75 ms) in area TE than in the other areas. Examples of each of these types of neurone with memory spans of approximately 24 h were found in each region. The mean memory span of recency neurones was significantly longer in perirhinal cortex than area TE. For familiarity neurones a significant mean response decrement took 4-8 min to develop, indicating a slow underlying plastic change, in contrast to the rapid change seen for recency and novelty neurones. The implications of these results are discussed in relation to the neuronal basis of recognition memory.

The kappa-opiate agonist U50488H decreases the entry of 45Ca into rat cortical synaptosomes by inhibiting N- but not L-type calcium channels.

The selective kappa-opiate agonist U50488H (1-100 microM) significantly reduced the uptake of 45Ca into cortical synaptosomes from the brain of the rat, in a time- and dose-dependent manner. In physiological medium, the maximum inhibition occurred after 2 min; this was approximately 55% (at 100 microM) and the IC50 was 80 nM. Nifedipine (1 microM) had no significant effect on the influx of Ca2+ in physiological medium (containing 5 mM K+), though, in fact, there was an approximately 20% decrease in the presence of 100 microM of drug. Nifedipine, however, did cause a significant blockade of the entry of 45Ca in medium containing 10 or 15 mM K+, demonstrating that L-type channels on synaptosomes were operational under depolarising conditions. Under these depolarising conditions, there was an additive inhibitory effect on entry of 45Ca into synaptosomes when U50488H (1 microM) and nifedipine (1 microM) were incubated together. Treatment of synaptosomes with omega-conotoxin (omega-CgTx, 0.5 microM) resulted in a 35% reduction in the uptake of 45Ca. omega-Conotoxin (0.5 microM) or naloxone (20 microM) abolished the inhibitory effect of U50488H on the uptake of 45Ca, but naloxone did not alter the blockade of L-type Ca2+ channels, caused by nifedipine. In conclusion, the data demonstrate that under depolarising conditions, there are functional L-type calcium channels on nerve endings in the CNS.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of receptor-mediated inhibition of 45Ca accumulation into synaptosomes.

1. The effects of alpha 2-adrenoceptor and kappa-opiate receptor activation on 45Ca accumulation into rat cortical synaptosomes were examined. 2. Clonidine (1 microM) and U50488H (1 microM) significantly reduced 45Ca accumulation under both resting (5 mM K+) and depolarizing (15-30 mM K+) conditions. 3. The inhibitory effects of the agonists on 45Ca accumulation into synaptosomes were enhanced in the presence of the Na(+)-Ca2+ exchange inhibitor sodium orthovanadate (vanadate, 2 mM), and were not present in mitochondrial preparations. 4. When the agonists were used together, their inhibitory effects were not additive but were, in fact, attenuated. 5. In the presence of the alpha 2-adrenoceptor antagonist idazoxan (1 microM), the inhibitory effect of U50488H on 45Ca accumulation was enhanced. A similar increase in the inhibitory effectiveness of clonidine was observed in the presence of naloxone (20 microM). 6. When synaptosomes were pretreated with 3-isobutyl-1-methylxanthine (IBMX, 0.5 mM), dibutyrylcyclic AMP (db-cyclic AMP, 10 microM) or 8-bromo-cyclic AMP (8Br-cyclic AMP, 10 microM), the inhibitory effects of clonidine and U50488H were abolished, suggesting that a decrease in cyclic AMP production is part of the receptor-effector coupling mechanism of both receptor systems. 7. The phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA, 0.05 microM) increased 45Ca accumulation but did not alter the inhibitory effects of clonidine or U50488H, thus showing that the effects of the agonists are not mediated by protein kinase C. 8. We conclude that alpha 2-adrenoceptor and Kappa-opiate receptor activation dramatically reduce 45Ca influx through Ca21 channels (e.g., by 50%), that there is a functional antagonism between the two receptor systems and that in both cases, the receptor effector mechanism involves a decrease in cyclic AMP production.

Quisqualate and carbachol-induced increases in intrasynaptosomal free calcium are mediated by different products of phospholipid hydrolysis.

The mechanisms by which quisqualate and carbachol increase intrasynaptosomal free calcium ([Ca2+]i) were studied in rat cortical synaptosomes. Quisqualate (0.01-100 microM) and carbachol (100-1000 microM) increased [Ca2+]i in Fura-2 acetoxymethyl ester (Fura-2 AM)-loaded synaptosomes. The resting level of [Ca2+]i was 118 nM. The maximum increase (55%) was produced by 10 microM quisqualate which had an EC50 of 0.2 microM. The maximum increase (28%) elicited by carbachol occurred at 1000 microM and the EC50 was 30 microM. The stimulatory effects of quisqualate on [Ca2+]i were blocked by heparin (100 I.U.) but not by staurosporine (1 microM), nifedipine (1 microM) or omega-conotoxin fraction GVIA (omega-CgTx) (0.5 microM). On the other hand, the effects of carbachol on [Ca2+]i were abolished by staurosporine, nifedipine or omega-CgTx but not by heparin. Carbachol (100 microM) also significantly increased 45Ca accumulation into either resting or K+ (30 mM)-depolarised synaptosomes and these effects were inhibited by staurosporine and nifedipine. Quisqualate (10 microM) had no effect on 45Ca accumulation under resting or depolarised conditions. When quisqualate and carbachol were used in combination, there were apparently additive effects on [Ca2+]i but not on 45Ca accumulation. It is concluded that carbachol increases [Ca2+]i by facilitating Ca2+ entry through L-type Ca2+ channels via a 1,2-diacylglycerol (DAG)-protein kinase C (PKC)-dependent pathway while quisqualate mobilizes Ca2+ from inositol 1,4,5-trisphosphate (IP3)-sensitive stores.

Effects of converting enzyme inhibitors: ramipril and enalapril on peptide action and sympathetic neurotransmission in the isolated heart.

The role of converting enzyme (CE) in heart function was studied by assessing the inhibition of CE activity in rat heart tissue and in isolated perfused hearts from rat, guinea-pig and rabbit (Langendorff technique). Angiotensin I (ANG I) added to the perfusate reduced coronary flow (FLO) in all species, increased force of contraction (CON) in rats, decreased CON in guinea-pigs and had no effect on CON in rabbits. In contrast, bradykinin (BK) increased FLO in all species, decreased CON in rats, increased CON in guinea-pigs and had no effect on CON in rabbits. The CE inhibitors ramipril (HOE498) 1 mg/kg and enalapril (MK421) 30 mg/kg given orally 1 h prior to killing of the animals inhibited the ANG I effects and potentiated the BK effects but had no effects on the action of ANG II. The same doses of the two CE inhibitors produced up to 24 h inhibition of CE activity measured biochemically in the heart after single oral doses. Electrical sympathetic stimulation of the cardiac nerves (SNS) in isolated rabbit heart resulted in an increase of HR and CON and in an initial decrease and subsequent increase of FLO. The effects of SNS on HR and FLO were significantly reduced following HOE498 (1 mg/kg) pretreatment. The results suggest that local CE is involved in the regulation of peptide effects in the heart, including the ANG II-mediated facilitation of neurotransmission.

Presynaptic alpha 2-adrenoceptor and kappa-opiate receptor occupancy promotes closure of neuronal (N-type) calcium channels.

Synaptosomes prepared from rat cerebral cortex by homogenization in isotonic sucrose and centrifugation on four-step discontinuous percoll density gradients were loaded with the fluorescent indicator fura-2 to allow measurement of intrasynaptosomal free calcium concentrations [( Ca2+]i). Incubation of fura-2 loaded synaptosomes with either the kappa-opiate agonist U-50,488H (0.1-100 microM) or the alpha 2-adrenoceptor agonist clonidine (0.1-100 microM), resulted in a dose-dependent reduction in [Ca2+]i and these changes were completely antagonised by prior inclusion of naloxone (20 microM) or idazoxan (RX781094) (2 microM) respectively. When the 1,4-dihydropyridine Ca2+-channel blocker nifedipine (1 microM) was incubated with synaptosomes for 1 min, there was a 17.0% decrease in [Ca2+]i and when it was combined with either U-50,488H (1 microM) or clonidine (1 microM) there was a reduction in [Ca2+]i of 35.0 and 48.1% respectively i.e. the effects were additive. The increases in the depression of [Ca2+]i produced by these drug combinations were antagonised by the inclusion of naloxone (20 microM) or idazoxan (2 microM) which resulted in decreases in free [Ca2+]i of 26.5 and 14.1% respectively. These data indicate that the effects of clonidine and U-50,488H are not mediated by L-type Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Kainate and quisqualate effects on rat presynaptic cortical receptors are metabotropic and non-additive.

The effects of quisqualate and kainate on synaptosomal inositol phosphate (InsP) labelling, 45Ca influx and intrasynaptosomal free calcium ([Ca2+]i) were investigated. Each agonist caused a concentration-dependent increase in both [Ca2+]i and InsP labelling: quisqualate, however, produced significantly larger responses in both parameters and at lower EC50 values. Neither quisqualate or kainate significantly affected 45Ca influx into synaptosomes, indicating that the observed increases in [Ca2+]i were due to mobilisation from intracellular stores. The concentration-dependent increases in [Ca2+]i promoted by quisqualate and kainate were monophasic, whereas the increases in InsP formation fitted well to a biphasic curve. The EC50 values suggest that both kainate and quisqualate initially mobilise calcium from inositol 1,4,5-trisphosphate (Ins 1,4,5-P3)-sensitive stores and that the resultant increases in [Ca2+]i will, above a certain threshold, promote further increases in InsP production by stimulation of Ca(2+)-dependent phospholipase C. When saturating concentrations of kainate and quisqualate were used in combination, the effects on both InsP labelling and [Ca2+]i were not additive but were slightly higher than those produced by kainate alone: combined administration of the two agonists had no effect on 45Ca influx. These results suggest that kainate acts as a partial agonist at the presynaptic quisqualate metabotropic glutamatergic receptor.

[A study on the change of myocardial neuropeptide Y release induced by electric stimulation and myocardial ischemia in guinea pig heart].

Electrical stimulation of the left stellate ganglion in guinea pig heart evoked a calcium-dependent, exocytotic release of neuropeptide Y (NPY). Stimulation after 10 min of global ischemia (S2), compared with control period stimulation (S1), had no significant effect on the NPY release. The release of NPY produced by the same stimulation after 20 min of ischemia was inhibited to certain extent (S2/S1: 0.72, P < 0.05), whereas the inhibition of NPY release disappeared after 5 min of reperfusion (with a S2/S1 of 1.01). Ischemia alone, without electric stimulation, did not apparently induce NPY release, suggesting that electrical stimulation may induce a calcium-dependent, exocytotic release of NPY. It is further suggested that the inhibition of NPY release may be produced by some metabolites and the abolishment of the inhibition after reperfusion may be due to washout of the metabolites.

Comparison of differential neuronal responsiveness for different regions of the prefrontal cortex in a conditional visual discrimination task.

To investigate neuronal processing during monkeys' performance of a visual conditional discrimination task, recordings were made from four areas of prefrontal cortex (ventromedial, orbitofrontal, dorsolateral and anterior cingulate) where lesions have been shown to produce impairment of such tasks. Of 1911 recorded neurons, 573 (31%) responded to elements of the task. This proportion was less than the 50% previously reported as responsive in temporal cortex under the same conditions, suggesting sparser encoding in prefrontal than temporal cortex. Of the responsive prefrontal neurons, 165 (29%) responded differently on the different types of trial, so signalling various types of information relevant to task performance and cognition. In line with recent lesion findings, in the dorsolateral region the incidence of such differentially responsive neurons was only an eighth that in the other regions. The relatively high incidence of neuronal responses that encoded a potential instruction cue rather than specific individual stimulus arrangements was consistent with the animals solving the task by using such information, though other neuronal responses could have enabled the task to have been solved by rote learning. Compared to temporal neurons, prefrontal responses more frequently coded information relating to the planned behavioural response rather than perceptual aspects of the task. Population differential response latencies were long (> approximately 225 ms) in prefrontal cortex. A comparison of such differential latencies between temporal and prefrontal cortex indicated that potential information flow was likely to be primarily from temporal to prefrontal cortex rather than vice versa.

Differential neuronal responsiveness in primate perirhinal cortex and hippocampal formation during performance of a conditional visual discrimination task.

Neuronal responses in the hippocampal formation, including the entorhinal cortex, have been compared with those in the inferior temporal cortex, including the perirhinal cortex, during performance by monkeys of a visual conditional discrimination task. In the task, the arrangement of three geometric shapes determined the correctness of either a left or right behavioural response according to a conditional rule. Neurons that responded differently to different types of trial were common (50% of the visually responsive neurons) in the entorhinal cortex, perirhinal cortex and area TE of the inferior temporal cortex, but significantly less common in the hippocampus (13%). This differential incidence suggests a more important role for the rhinal cortices and area TE than for the hippocampus in this task. Based on the neuronal responses, arguments are advanced that the animals probably solved the task by a strategy that did not require spatial or hippocampal processing. Thus, of the differential responses, those that would allow the animals to solve the task by using a conditional rule and so avoid spatial processing were twice as common (37%) as those allowing solution to be by selection of a particular spatially directed response to each arrangement of shapes (19%). Moreover, the differential latencies of responses that allowed the task to be solved by a conditional rule were shorter (< approximately 165 ms), and hence processing was faster, than those that provided information about particular individual types of trial ( approximately 195 ms). Even so, hippocampal responsiveness in the conditional task was differentially enhanced when compared with that during a recognition memory task, and the neuronal responses potentially allow the animal to employ a second, alternative strategy that might be expected to depend on hippocampal processing.

Measles vaccine in the People's Republic of China.

The strains of measles virus and the method used in the production of further attenuated live-virus vaccine in the People's Republic of China were studied. Observation of clinical reactions and serologic responses to immunization with Shanghai-191 measles vaccine, which is produced on a large scale with a locally isolated viral strain, revealed that this vaccine is adequately safe and immunogenic. Epidemiologic data showed a significant decrease in measles-associated morbidity after the introduction of mass vaccination in 1965. The duration of immunity induced by Shanghai-191 measles vaccine was studied for eight years in a region in which interference due to natural measles infection had been minimized by mass vaccination of children. Although immunity appeared to persist for at least eight years, the results suggested that primary vaccination does not confer lifelong immunity. Reactions and antibody responses to this vaccine were compared with those to two vaccines from abroad, the Schwarz and Leningrad-16 strains. The hemagglutination-inhibiting (HAI) antibody titer induced by Shanghai-191 vaccine was higher than that induced by Leningrad-16 vaccine and lower than that induced by Schwarz vaccine; however, these differences were not significant. Preliminary studies on the preparation of measles vaccine in human diploid cells have yielded promising results.

Central effects of converting enzyme inhibitors.

Evidence has accumulated that systemic administration of converting enzyme inhibitors (CEI) such as captopril, MK 421 or SA 446 not only produces an inhibition of the plasma renin angiotensin system (RAS), but also of the RAS in various target organs which are relevant for blood pressure (BP) regulation. A potential target organ is the brain, where a local CE inhibition could contribute to the BP lowering action of CEI. CE in the brain can be inhibited by intracerebroventricular (i.c.v.) injection of CEI as evidenced by an inhibition of the pressor and drinking responses to i.c.v. angiotensin I (ANG I) or renin and by potentiation of the pressor responses to i.c.v. bradykinin. Site of the inhibition is not only the cerebrospinal fluid but also periventricular brain tissue such as the hypothalamus. I.c.v. injection of captopril at doses which inhibit brain CE but do not leak into the peripheral blood were shown to lower BP in conscious stroke-prone spontaneously hypertensive rats (SHRSP), but not in normotensive Wistar Kyoto (WKY) controls. Acute peripheral administration of CEI can produce an inhibition of brain CE. This was shown by an attenuation of the drinking responses to i.c.v. ANG I and renin and by direct measurements of CE activity in brain tissue. Chronic oral treatment with CEI produces changes of brain RAS parameters which suggest an inhibition of ANG II formation in the brain.

Regulation of release and function of neuropeptides in the heart.

The mechanisms regulating the release of enkephalins and neuropeptide Y (NPY) from cardiac nerve fibers were studied in the isolated perfused guinea pig heart. Besides NPY, all peptides of the proenkephalin B family were found to be stored in the adrenergic nerve endings. Following electrical stimulation of the accelerans nerve, but not of the vagus nerve, the peptides were detected in the perfusate indicating their release from sympathetic nerve endings. The release of both enkephalins and NPY was enhanced by phentolamine or yohimbine and diminished by cocaine. Thus, sympathetic amines may directly regulate the release of their cotransmitters via a presynaptic alpha-adrenergic mechanism. Enkephalins exerted a weak negative inotropic effect that appeared to be mediated by opioid receptors of the kappa type. Neuropeptide Y is a potent vasoconstrictor and may be involved in the regulation of coronary blood flow.

Regulation of calcium concentrations in synaptosomes: alpha 2-adrenoceptors reduce free Ca2+ by closure of N-type Ca2+ channels.

The relationship between intrasynaptosomal total (CaT) and free ([Ca2+]i) calcium and 45Ca accumulation was studied under physiological and K(+)-depolarised conditions in rat cortical synaptosomes. Under physiological conditions, CaT (10.7 mM) was approximately 10,000 times higher than [Ca2+]i (118 nM), showing that there is a large reservoir of sequestered calcium in synaptosomes. 45Ca accumulation was rapid (initial rate, 3.4 nmol/mg protein/min), substantial (7 nmol/mg protein in 2 min), and depolarisation dependent, and reached equilibrium after 5 min. At equilibrium, only 10% of CaT was freely exchangeable. This pool was much larger than the free Ca2+ pool. CaT, [Ca2+]i, and 45Ca accumulations were directly related to the Ca2+ concentration in the buffer, suggesting that [Ca2+]i is not highly conserved but is maintained by simple equilibria between the various pools. Clonidine reduced 45Ca accumulation in a time- and dose-dependent manner. Maximum inhibition (40% at 100 microM) occurred at 2 min and the IC50 was 80 nM. The reduction caused by clonidine (1 microM) reached equilibrium after 5 min, but this equilibrium value was lower than in controls, suggesting that clonidine changes the exchangeable Ca2+ pool size. The effects of clonidine (1 microM) on [Ca2+]i (26% reduction) and on 45Ca accumulation (24% reduction) were most apparent under physiological conditions. However, while it was not dependent on depolarisation, it did not occur in physiological buffer containing low K+ concentration (0.1-1 mM). The inhibitory effect of clonidine on 45Ca accumulation is receptor mediated as it was antagonised by idazoxan (1 microM).(ABSTRACT TRUNCATED AT 250 WORDS)