Neurotransmitter inhibition of calcium currents (ICa) can be relieved by large depolarizing prepulses. This effect has been postulated to be due either to the voltage-dependent unbinding of an inhibitory molecule from the channel or to a slow voltage-dependent gating step intrinsic to the modulated channel. According to the first hypothesis, the rate of reinhibition (reblock) following a depolarizing prepulse should depend on the concentration of active inhibitory molecules and thus should increase with the extent of inhibition. To distinguish between these models we examined the actions of norepinephrine (NE) and somatostatin (SS) on high-threshold calcium currents in chick sympathetic ganglia, using whole-cell voltage-clamp methods. As previously described in other systems, both NE and SS inhibit omega- conotoxin-sensitive N-type Ca2+ current in a voltage-dependent manner. Pertussis toxin (PTX) pretreatment prevents the inhibition of the current, while replacing GTP in the patch pipette with GTP-gamma-S results in irreversible inhibition, consistent with the involvement of a PTX-sensitive G-protein. The inhibitory responses to NE and SS are not additive, suggesting that they act at a common locus. The inhibitory response to repeated applications of NE or SS desensitizes, with little evidence for cross desensitization. The inhibition of ICa is relieved by a 15 msec prepulse to +100 mV. Following repolarization to -80 mV, ICa slowly reblocks. During prolonged applications of NE or SS the extent of inhibition decreases due to desensitization and reblock kinetics are significantly slowed (time constant increases from 60 msec to > 100 msec for both NE and SS). These results are well fit by a quantitative model in which the kinetics of reblock reflect the binding of an inhibitory molecule to the channel.

Previous studies demonstrated that hydrogen peroxide (H(2)O(2)) is a tumor promoter in the rat liver epithelial cell line T51B. We investigated the pathway linking H(2)O(2) to tumor promotion. H(2)O(2) can directly induce tyrosine phosphorylation of epidermal growth factor receptor (EGFR). H(2)O(2) and epidermal growth factor exerted similar effects on the induction of early growth response genes, disruption of gap junction communication, triggering of calcium inflow, and promotion of transformation. Furthermore, the effect of H(2)O(2) on tumor promotion was blocked by abrogation of EGFR activation. Our results suggested that tumor promotion by H(2)O(2) is mediated mainly through activation of EGFR in T51B cells.

1. Membrane currents of horizontal cells isolated from the retina of the turtle, Pseudemys, were characterized by the whole-cell patch-clamp technique. 2. Four membrane currents were identified: an anomalous rectifier blocked by barium, a transient A-current, a sustained L-type calcium current enhanced by Bay K 8644, and a fast, tetrodotoxin-sensitive sodium current. Each of these four currents was found in both horizontal cell somata and axon terminals. 3. The current-voltage relations of axon terminals and somata were similar, but, in the normal operating range of the cell (-30 to -50 mV), the mean slope resistance of the axon terminal was higher (1.38 G omega) than that of the soma (0.26 G omega). 4. Exposure to either glutamate, kainate, or quisqualate induced a sustained inward current in horizontal cell axon terminals. The reversal potential for this current was -3 mV when tested with voltage steps and +9.1 mV when measured by a voltage ramp. The same horizontal cells were insensitive to N-methyl-D-aspartate. 5. A continuum model was developed to compute the degree of signal transfer between a horizontal cell body and its axon terminal. The model consisted of a network of electrically coupled somata that communicates with a network of electrically coupled axon terminals through the connecting axons. The specific membrane resistances used for the model derived from the patch-clamp measures. 6. We computed the voltage change elicited in either the layer of somata or of axon terminals by a static light stimulus of arbitrary dimensions. The amplitude of a spot response as a function of its radius was given by the weighted sum of two Bessel functions with different space constants. 7. The computed responses of the cell body were dominated by the Bessel function with the smaller space constant, whereas those of the axon terminal depended primarily on the Bessel function with the larger space constant. 8. The model predicts that, in contrast to the findings in teleost retina, there is little signal transfer between the somata and axon terminals of horizontal cell in the turtle retina.

1. Somatostatin produces a voltage-dependent inhibition of N-type Ca2+ current in chick sympathetic neurons. Pretreatment of chick sympathetic ganglion neurons with protein kinase C (PKC) activators has no effect on calcium current (ICa) but reduces the inhibition of ICa by somatostatin. 2. The effects of the alkaloid PKC activator (-)-indolactam V were indistinguishable from those of 4 beta-phorbol-12-myristate-13-acetate (4 beta-PMA). The inactive isomers (+)-indolactam V and 4 alpha-PMA did not alter the modulation of ICa by somatostatin. 3. Modulation of ICa by somatostatin desensitizes, with a time for half desensitization of approximately 3 min. PKC activation mimics the normal desensitization process in that responses to 30 nM somatostatin are inhibited to a greater extent than are responses to 1 microM somatostatin. 4. PKC appears to act at the level of the somatostatin receptor or receptor-G protein interaction because PKC activation does not alter Ca2+ current inhibition in response to a nonhydrolyzable analog of GTP, GTP-gamma-S, which directly activates G proteins. 5. The specific PKC inhibitor calphostin C largely reverses the effects of phorbol esters, but does not slow the normal rate of desensitization of somatostatin responses. This indicates that PKC is not involved in the homologous desensitization of the somatostatin receptor. 6. Neither substance P, which activates PKC in these cells, nor arachidonic acid, another PKC activator, altered the action of somatostatin on ICa.