In all preparations studied, a characteristic distinction of Sr¥versus Ca¥-mediated synaptic transmission is a reduced early (or phasic) component and an augmented late (or asynchronous) tail of individual miniature-like events called 'late release'. Thus, transmission in Sr¥ is desynchronized with respect to the normal situation. Understanding this non-physiological phenomenon may entail important insights into the mechanisms which under physiological conditions control the kinetics of evoked transmitter release. Thus, have proposed that preferential activation by Sr¥ of late, asynchronous release is due to binding at a secondary, highaffinity divalent cation binding site, where the action of Sr¥ is stronger than that of Ca¥. A high-affinity site in addition to the putative low-affinity receptor involved in synchronous release (cf. Heidelberger et al. 1994) has also been postulated in order to account for use-dependent synaptic facilitation (Yamada & Zucker, 1992). The twobinding-site hypothesis has gained indirect support from experiments on excitatory synapses lacking the presynaptic Ca¥-binding protein synaptotagmin I, where phasic release was suppressed and asynchronous release was intact or increased (Geppert et al. 1994). Alternatively, it has been suggested that a single releaseinducing binding site may suffice to explain Sr¥-induced late release if buffering, extrusion or sequestration are assumed to be less efficient for Sr¥ than for Ca¥ so that free Sr¥ concentration remains elevated for longer times

Immunological stimulation of rat mucosal-type mast cells (RBL-2H3 line) by clustering of their Fcepsilon receptors (FcepsilonRI) causes a rapid and transient increase in free cytoplasmic Ca(2+) ion concentration ([Ca(2+)](i)) because of its release from intracellular stores. This is followed by a sustained elevated [Ca(2+)](i), which is attained by Ca(2+) influx. Because an FcepsilonRI-induced increase in the membrane permeability for Na(+) ions has also been observed, and secretion is at least partially inhibited by lowering of extracellular sodium ion concentrations ([Na(+)](o)), the operation of a Na(+)/Ca(2+) exchanger has been considered. We found significant coupling between the Ca(2+) and Na(+) ion gradients across plasma membranes of RBL-2H3 cells, which we investigated employing (23)Na-NMR, (45)Ca(2+), (85)Sr(2+), and the Ca(2+)-sensitive fluorescent probe indo-1. The reduction in extracellular Ca(2+) concentrations ([Ca(2+)](o)) provoked a [Na(+)](i) increase, and a decrease in [Na(+)](o) results in a Ca(2+) influx as well as an increase in [Ca(2+)](i). Mediator secretion assays, monitoring the released beta-hexosaminidase activity, showed in the presence of extracellular sodium a sigmoidal dependence on [Ca(2+)](o). However, the secretion was not affected by varying [Ca(2+)](o) as [Na(+)](o) was lowered to 0.4 mM, while it was almost completely inhibited at [Na(+)](o) = 136 mM and [Ca(2+)](o) < 0.05 mM. Increasing [Na(+)](o) caused the secretion to reach a minimum at [Na(+)](o) = 20 mM, followed by a steady increase to its maximum value at 136 mM. A parallel [Na(+)](o) dependence of the Ca(2+) fluxes was observed: Antigen stimulation at [Na(+)](o) = 136 mM caused a pronounced Ca(2+) influx. At [Na(+)](o) = 17 mM only a slight Ca(2+) efflux was detected, whereas at [Na(+)](o) = 0.4 mM no Ca(2+) transport across the cell membrane could be observed. Our results clearly indicate that the [Na(+)](o) dependence of the secretory response to FcepsilonRI stimulation is due to its influence on the [Ca(2+)](i), which is mediated by a Na(+)-dependent Ca(2+) transport.

The effect of benzodiazepines (BZs) on GABA(A)-ergic synaptic responses depends on the control receptor occupancy: the BZ-induced enhancement of receptor affinity can lead to greater peak amplitudes of quantal responses only when, under normal conditions, receptors are not fully saturated at peak. Based on this fact, receptor occupancy at the peak of spontaneous miniature inhibitory postsynaptic currents (mIPSCs) has been assessed in various mammalian neuronal preparations. To use the same principle with compound (or multiquantal), action potential-evoked IPSCs, complications introduced by quantal asynchrony in conjunction with the BZ-induced increase in the decay time of the quantal responses have to be overcome. We used a simple analytic convolution model to calculate expected changes in the rise time and amplitude of postsynaptic currents when the decay time constant, but not the peak amplitude, of the underlying quantal responses is increased, this being the expected BZ effect at saturated synapses. Predictions obtained were compared with the effect of the BZ flunitrazepam on IPSCs recorded in paired pre- and postsynaptic whole cell voltage-clamp experiments on striatal neurons in cell culture. In 22 pairs, flunitrazepam (500 nM) reliably prolonged the decay of IPSCs (49 +/- 19%, mean +/- SE) and in 18 of 22 cases produced an enhancement in their peak amplitude that varied markedly between 3 and 77% of control (26.0 +/- 5.3%). The corresponding change in rise time, however (+0.38 +/- 0.11 ms, range -0.8 to +1.3 ms) was far smaller than calculated for the observed changes in peak amplitude assuming fixed quantal size. Because therefore an increase in quantal size is required to explain our findings, postsynaptic GABA(A) receptors were most likely not saturated during impulse-evoked transmission at these unitary connections. The peak amplitudes of miniature IPSCs in these neurons were also increased by flunitrazepam (500 nM, +26.8 +/- 6.6%), and their decay time constant was increased by 26.3 +/- 7.3%. Using these values in our model led to a slight overestimate of the change in compound IPSC amplitude (+28 to +30%).

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