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Biomedical subjects

R Ravin

Publications and source records attributed to R Ravin.

10 recordsLinked to original sources
PubMed2001

Differentiation of embryonic stem cells to insulin-secreting structures similar to pancreatic islets.

Although the source of embryonic stem (ES) cells presents ethical concerns, their use may lead to many clinical benefits if differentiated cell types can be derived from them and used to assemble functional organs. In pancreas, insulin is produced and secreted by specialized structures, islets of Langerhans. Diabetes, which affects 16 million people in the United States, results from abnormal function of pancreatic islets. We have generated cells expressing insulin and other pancreatic endocrine hormones from mouse ES cells. The cells self-assemble to form three-dimensional clusters similar in topology to normal pancreatic islets where pancreatic cell types are in close association with neurons. Glucose triggers insulin release from these cell clusters by mechanisms similar to those employed in vivo. When injected into diabetic mice, the insulin-producing cells undergo rapid vascularization and maintain a clustered, islet-like organization.

Animals↗
PubMed2000

Novel mechanism for presynaptic inhibition: GABA(A) receptors affect the release machinery.

Presynaptic inhibition is produced by increasing Cl(-) conductance, resulting in an action potential of a smaller amplitude at the excitatory axon terminals. This, in turn, reduces Ca(2+) entry to produce a smaller release. For this mechanism to operate, the "inhibitory" effect of shunting should last during the arrival of the "excitatory" action potential to its terminals, and to achieve that, the inhibitory action potential should precede the excitatory action potential. Using the crayfish neuromuscular preparation which is innervated by one excitatory axon and one inhibitory axon, we found, at 12 degrees C, prominent presynaptic inhibition when the inhibitory action potential followed the excitatory action potential by 1, and even 2, ms. The presynaptic excitatory action potential and the excitatory nerve terminal current (ENTC) were not altered, and Ca(2+) imaging at single release boutons showed that this "late" presynaptic inhibition did not result from a reduction in Ca(2+) entry. Since 50 microM picrotoxin blocked this late component of presynaptic inhibition, we suggest that gamma-aminobutyric acid-A (GABA(A)) receptors reduce transmitter release also by a mechanism other than affecting Ca(2+) entry.

Action Potentials↗
PubMed1999

Simultaneous measurement of evoked release and [Ca2+]i in a crayfish release bouton reveals high affinity of release to Ca2+.

The opener neuromuscular junction of crayfish was used to determine the affinity of the putative Ca2+ receptor(s) responsible for evoked release. Evoked, asynchronous release, and steady-state intracellular Ca2+ concentration, [Ca2+]ss, were measured concomitantly in single release boutons. It was found that, as expected, asynchronous release is highly correlated with [Ca2+]ss. Surprisingly, evoked release was also found to be highly correlated with [Ca2+]ss. The quantal content (m) and the rate of asynchronous release (S) showed sigmoidal dependence on [Ca2+]ss. The slope log m/log [Ca2+]ss varied between 1.6 and 3.3; the higher slope observed at the lower [Ca2+]o. The slope log S/log [Ca2+]ss varied between 3 and 4 and was independent of [Ca2+]o. These results are consistent with the assumption that evoked release is controlled by the sum of [Ca2+]ss and the local elevation of Ca2+ concentration near the release sites resulting from Ca2+ influx through voltage-gated Ca2+ channels (Y). On the basis of the above, we were able to estimate Y. We found Y to be significantly <10 microM even for [Ca2+]o = 13.5 mM. The dissociation constant (Kd) of the Ca2+ receptor(s) associated with evoked release was calculated to be in the range of 4-5 microM. This value of Kd is similar to that found previously for asynchronous release.

Animals↗
PubMed1999

Partial uncoupling of neurotransmitter release from [Ca2+]i by membrane hyperpolarization.

The dependence of evoked and asynchronous release on intracellular calcium ([Ca2+]i) and presynaptic membrane potential was examined in single-release boutons of the crayfish opener neuromuscular junction. When a single bouton was depolarized by a train of pulses, [Ca2+]i increased to different levels according to the frequency of stimulation. Concomitant measurements of evoked release and asynchronous release, from the same bouton, showed that both increased in a sigmoidal manner as a function of [Ca2+]i. When each of the depolarizing pulses was immediately followed by a hyperpolarizing pulse, [Ca2+]i was elevated to a lesser degree than in the control experiments, and the rate of asynchronous release and the quantal content were reduced; most importantly, evoked quantal release terminated sooner. The diminution of neurotransmitter release by the hyperpolarizing postpulse (HPP) could not be entirely accounted for by the reduction in [Ca2+]i. The experimental results are consistent with the hypothesis that the HPP reduces the sensitivity of the release machinery to [Ca2+]i, thereby not only reducing the quantal content but also terminating the quantal release process sooner.

Animals↗
PubMed1997

Simultaneous measurement of intracellular Ca2+ and asynchronous transmitter release from the same crayfish bouton.

1. A technique has been developed to monitor neurotransmitter release simultaneously with intracellular Ca2+ concentration ([Ca2+]i) in single release boutons whose diameters range from 3 to 5 microns. 2. Using this technique, we have found a highly non-linear relationship between the rate of asynchronous release and [Ca2+]i. The Hill coefficient lies between 3 and 4. 3. The affinity (Kd) of the putative release-related Ca2+ receptor for asynchronous release was calculated to be in the range of 2-4 microM. 4. The same range of values of Hill coefficient and Kd were obtained when [Ca2+]i was elevated both by bath application of ionomycin and by repetitive stimulation at high frequency. 5. Our results show that the Ca2+ receptor(s) associated with asynchronous release exhibits high affinity for Ca2+.

Animals↗
PubMed1996

Glutamate depresses release by activating non-conventional glutamate receptors at crayfish nerve terminals.

The present study shows that release of glutamate from crayfish nerve terminals is inhibited at low depolarizing current pulses by glutamate, N-methyl-D-aspartate (NMDA) and quisqualate. These agonists elicit inhibitory effects at concentrations as low as 10(-8) M (quisqualate) and 10(-7) M (glutamate and NMDA). The NMDA-mediated inhibition is blocked by (+/-)-2-amino-5-phosphonovaleric acid (APV). The quisqualate-mediated inhibition is blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Both CNQX and APV are needed to block glutamate-mediated inhibition. The inhibition of release is not accompanied by a detectable change in presynaptic membrane conductance at the secondary branch. Using fura-2, Ca2+ accumulation during repetitive stimulation (100 Hz) was monitored in single release boutons. Inhibition of release, elicited by 10(-4) M glutamate, was not associated with a reduction in the accumulation of Ca2+. We show that the glutamate released from a single or a few release boutons during normal activity acts similarly to glutamate added externally, i.e. it inhibits its own release.

2-Amino-5-phosphonovalerate↗
PubMed1994

Glutamate and N-methyl-D-aspartate affect release from crayfish axon terminals in a voltage-dependent manner.

In the crayfish neuromuscular junction, the excitatory transmitter is glutamate. The present study shows that at concentrations as low as 5 x 10(-7) M, glutamate affects the depolarization-evoked release of neurotransmitter. Furthermore, the effect of glutamate on release is voltage-dependent and depends on the level of the depolarizing pulse. Nerve terminals were exposed to 5 x 10(-7) M tetrodotoxin and then depolarized to different levels by a macropatch electrode. Depending on the amplitude of the depolarizing pulse, glutamate (5 x 10(-7) to 1 x 10(-5) M) had a dual effect on release. At small depolarizing pulses, glutamate reduced release, whereas at large depolarizing pulses, it enhanced it. Glutamate at 10(-6) M had no significant effect on action-potential-induced release. At 10(-4) M glutamate, the action-potential-induced release was always inhibited. N-Methyl-D-aspartate was found to mimic one of the effects of glutamate: N-methyl-D-aspartate (10(-7) to 10(-5) M) reduced release at small depolarizing pulses but had no effect with larger depolarizations. 2-Amino-5-phosphonovaleric acid blocked the effect of N-methyl-D-aspartate.

2-Amino-5-phosphonovalerate↗
PubMed1990

The biphasic effect of amrinone on tension development in newborn mammalian myocardium.

The effect of the bipyridine compound, amrinone, on tension generation in neonatal and adult myocardium was studied over the concentration range 30-500 micrograms/mL. Increasing concentrations of amrinone caused a monotonic increase in twitch tension and the rate of tension development in adult papillary muscles. In contrast, lower concentrations of amrinone (30 and 100 micrograms/mL) caused a decrease in twitch tension and dP/dt in newborn papillary muscles, whereas 500 micrograms/mL amrinone caused a significant increase in both parameters in the younger age group. Lactic acid, used to dissolve amrinone, was shown to have no effect on tension development. Half relaxation time was decreased in adult preparations at all concentrations of amrinone. In comparison, the decrease in half relaxation time produced by amrinone in the newborn was significant only at a concentration of 500 micrograms/mL. Action potential duration in the newborn was significantly shortened by 30 micrograms/mL amrinone. In voltage clamp experiments, 30 micrograms/mL amrinone was shown to have no effect on tension accompanying two second voltage clamp steps to the plateau potential in newborn myocardium. Developed tension at 400 ms into the clamp step, final tension, and the ratio of early peak tension to final tension were all unchanged by the low concentration of amrinone. In contrast, 500 micrograms/mL amrinone in the newborn increased tension at 400 ms and final tension but had no effect on the ratio of early peak tension to final tension. These results suggest that the negative inotropic effect of lower concentrations of amrinone on neonatal myocardium is the result of changes in action potential configuration and not a true alteration in basic mechanisms of intracellular Ca2+ regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1980

Development of a multihospital pharmacy quality assurance program.

Seven community hospitals have worked cooperatively for 18 months to develop an initial hospital pharmacy quality assurance program. Auditing criteria were developed for nine service areas corresponding to the model program developed by the American Society of Hospital Pharmacists. Current plans are to implement and modify this program as required at each participating hospital. Follow-up programs will also be essential to a functional, ongoing program, and these will be developed in the future.

Interinstitutional Relations↗