Molecular basis for pacemaker cells in epithelia

Intercellular signaling is highly coordinated in excitable tissues such as heart, but the organization of intercellular signaling in epithelia is less clear. We examined Ca(2+) signaling in hepatoma cells expressing the hepatocyte gap junction protein connexin32 (cx32) or the cardiac gap junction protein cx43, plus a fluorescently tagged V(1a) vasopressin receptor (V(1a)R). Release of inositol 1,4,5-trisphosphate (InsP(3)) in wild type cells increased Ca(2+) in the injected cell but not in neighboring cells, while the Ca(2+) signal spread to neighbors when gap junctions were expressed. Photorelease of caged Ca(2+) rather than InsP(3) resulted in a small increase in Ca(2+) that did not spread to neighbors with or without gap junctions. However, photorelease of Ca(2+) in cells stimulated with low concentrations of vasopressin resulted in a much larger increase in Ca(2+), which spread to neighbors via gap junctions. Cells expressing tagged V(1a)R similarly had increased sensitivity to vasopressin, and could signal to neighbors via gap junctions. Higher concentrations of vasopressin elicited Ca(2+) signals in all cells. In cx32 or cx43 but not in wild type cells, this signaling was synchronized and began in cells expressing the tagged V(1a)R. Thus, intercellular Ca(2+) signals in epithelia are organized by three factors: 1) InsP(3) must be generated in each cell to support a Ca(2+) signal in that cell; 2) gap junctions are necessary to synchronize Ca(2+) signals among cells; and 3) cells with relatively increased expression of hormone receptor will initiate Ca(2+) signals and thus serve as pacemakers for their neighbors. Together, these factors may allow epithelia to act in an integrated, organ-level fashion rather than as a collection of isolated cells.

Vasodilator activity of crude methanolic extract of Gentiana kokiana Perr. et Song. (Gentianaceae)

Gentiana kokiana Perr. et Song. is a plant employed in the traditional medicine of Tuscany (Italy) as antihypertensive remedy. The aim of this work was to evaluate a possible vascular action of the plant and to investigate its mechanism of action. The methanolic extract of roots showed an endothelium-independent vasodilator activity in aortic rings pre-contracted by norepinephrine (NE) 3 microM and a marked depression of the contracturant responses induced by KCl and caffeine, and by NE, both in Tyrode solution and Ca2+-free Tyrode solution. An action on the Ca2+-extracellular influx was discarded, while release or uptake mechanisms of the sarcoplasmic reticulum were hypothesized. However, the incapacity of cyclopiazonic acid (20 microM), a blocker of Ca2+/ATPase of the sarcoplasmic reticulum, to reduce the vasodilator action of the extract allowed to exclude the involvement of such a mechanism. A possible involvement of the ryanodine-sensitive Ca2+ channels is suggested.

Regulatory and spatial aspects of inositol trisphosphate-mediated calcium signals

Hormones that act to release Ca2+ from intracellular stores initiate a signaling cascade that culminates in the production of inositol 1,4,5-trisphosphate (InsP3). The Ca2+ response mediated by InsP3 is not a sustained increase in the cytosolic Ca2+ concentration, but rather a series of periodic spikes that manifest as waves in larger cells. In vitro studies have determined that the key positive feedback parameter driving spikes and waves is a highly localized direct Ca(2+)-activation of InsP3-gated Ca2+ channels. Advances in fluorescent Ca2+ imaging have facilitated the resolution of individual positive feedback units. These studies have revealed that there are several modes of channel coupling underlying global Ca2+ signals; single channel openings or Ca2+ "blips," synchronized clusters of channels or Ca2+ "puffs," and cell wide calcium waves. It appears that the channel clusters that produce Ca2+ puffs are synchronized by the highly localized positive feedback that was predicted by the in vitro studies of channel regulation. Localization of InsP3-induced Ca2+ signals has been shown to be important for activation of several cellular processes including uni-directional salt flow and mitochondrial activation.

Ubiquitous molecular substrates for associative learning and activity-dependent neuronal facilitation

Recent evidence suggests that many of the molecular cascades and substrates that contribute to learning-related forms of neuronal plasticity may be conserved across ostensibly disparate model systems. Notably, the facilitation of neuronal excitability and synaptic transmission that contribute to associative learning in Aplysia and Hermissenda, as well as associative LTP in hippocampal CA1 cells, all require (or are enhanced by) the convergence of a transient elevation in intracellular Ca2+ with transmitter binding to metabotropic cell-surface receptors. This temporal convergence of Ca2+ and G-protein-stimulated second-messenger cascades synergistically stimulates several classes of serine/threonine protein kinases, which in turn modulate receptor function or cell excitability through the phosphorylation of ion channels. We present a summary of the biophysical and molecular constituents of neuronal and synaptic facilitation in each of these three model systems. Although specific components of the underlying molecular cascades differ across these three systems, fundamental aspects of these cascades are widely conserved, leading to the conclusion that the conceptual semblance of these superficially disparate systems is far greater than is generally acknowledged. We suggest that the elucidation of mechanistic similarities between different systems will ultimately fulfill the goal of the model systems approach, that is, the description of critical and ubiquitous features of neuronal and synaptic events that contribute to memory induction.

Inositol 1,4,5-trisphosphate and calcium regulate the calcium channel function of the hepatic inositol 1,4,5-trisphosphate receptor

The regulation of the inositol 1,4,5-trisphosphate (IP3) receptor in liver was analyzed using a novel superfusion method. Hepatic microsomes were loaded with 45Ca2+, and superfused at high flow rates to provide precise control over IP3 and Ca2+ concentrations ([Ca2+]) and to isolate 45Ca2+ release from reuptake. 45Ca2+ release was dependent on both [Ca2+] and IP3. The initial rate of 45Ca2+ release was a biphasic function of [Ca2+], increasing as [Ca2+] approached 3 microM but decreasing at higher concentrations, suggesting that the hepatic IP3 receptor is regulated by [Ca2+] at two sites, a high affinity potentiation site and a low affinity inhibitory site. The relationship between initial rates and IP3 concentration was steep (Hill coefficient of 3.4), suggesting that activation of the calcium channel requires binding of at least 3 IP3 molecules. IP3 concentrations above 10 microM produced rapid decay of release rates, suggesting receptor inactivation. Superfusion with 10 microM IP3 under conditions that minimize calcium release ([Ca2+] < 1 nM) inhibited 45Ca2+ release in response to subsequent stimulation (400 nM Ca2+). These data suggest sequential positive and negative regulation of the hepatic IP3 receptor by cytosolic calcium and by IP3, which may underlie hepatocellular propagation of regenerative, oscillatory calcium signals.

Characterization of the co-agonist effects of strontium and calcium on myo-inositol trisphosphate-dependent ion fluxes in cerebellar microsomes

Using sheep cerebellum microsomes previously loaded with 45Ca2+ or 90Sr2+, we measured the dependence of inositol 1,4,5-trisphosphate (InsP3)-induced efflux of these ions on Ca2+ or Sr2+ on the cytosolic side. At a low InsP3 concentration, Ca2+ in the submicromolar range only poorly activated 45Ca2+ or 90Sr2+ efflux, and higher Ca2+ concentrations were inhibitory. In contrast, Sr2+ in the micromolar range activated release efficiently, while only very high Sr2+ concentrations were inhibitory. Experiments were repeated in the presence of a high InsP3 concentration, which allowed increasing free Ca2+ to micromolar concentrations without inducing complete inhibition of the InsP3-dependent efflux. Under these conditions, micromolar Ca2+ was found to activate efflux to a large extent, similar to that previously found with Sr2+. Optimal activation by Ca2+ of the InsP3-dependent channel occurs at micromolar rather than submicromolar free Ca2+ concentrations, but at too low an InsP3 concentration, Ca(2+)-induced activation is counteracted by Ca(2+)-induced inactivation. Separate measurements of [3H]-InsP3 binding at a low concentration showed that Sr2+ and Ca2+ did not enhance the amount of bound [3H]-InsP3, implying that the activating effect of Sr2+ and Ca2+ in cerebellar microsomes is mediated by an increase in the channel opening probability and not by an increase in the receptor's affinity for InsP3. A similar relationship also holds in the case of the activating effect of nucleotides.

Stimulation of HIV expression by intracellular calcium pump inhibition

We have studied the role of intracellular calcium sequestration on human immunodeficiency virus (HIV) production by latently infected T-lymphocytic cells. Inhibition of the sarco-endoplasmic reticulum-type calcium transport ATPases by thapsigargin or cyclopiazonic acid induced activation of HIV production in the CEM-derived ACH-2 cells. An approximately 50% depletion of the thapsigargin-sensitive calcium pools as measured fluorimetrically of Indo-loaded cells fully activated virus production. Viral activation was manifest by increases in soluble viral core p24 production, increases in cellular immunofluorescent staining for viral antigens, and increased viral transcription as measured by HIV long terminal repeat-directed expression of the chloramphenicol acetyltransferase reporter gene. Virus induction could be blocked in a dose-dependent manner by the calcium channel blocker econazole. Virus production by the Jurkat-derived HIV-1-inducible J1.1 cells was not significantly stimulated by thapsigargin. These data indicate that intracellular calcium pool function is involved in the control of the transcription of proviral HIV in a cell type-specific manner within the T-lymphoid lineage and that ACH-2 cells represent a useful model for the study of calcium dependent activation of the transcription of proviral HIV.