Ca Trigger Research Articles

The structure of human MRP8, a member of the S100 calcium-binding protein family, by MAD phasing at 1.9 A resolution.

The structure of human MRP8 in the calcium-bound form was determined at 1.9 A resolution by X-ray crystallography. The structure was initially solved by MAD phasing of an ytterbium-substituted crystal and was refined against data obtained from a Ca(2+)-bound crystal. The dimeric form of MRP8 was stabilized by hydrophobic interactions between mutually wrapped helices. There were two EF-hand motifs per monomer and each EF-hand bound one Ca(2+) with a different affinity [the affinity of the C-terminal EF-hand (EF-2) for Ca(2+) was stronger than that of the N-terminal EF-hand (EF-1)]. Furthermore, replacement with Yb(3+) occurred in the C-terminal EF-hand only, suggesting a more flexible nature for EF-2 than for EF-1. This, combined with previous observations that the helix in EF-2 (helix III) undergoes a large conformational change upon calcium binding, suggests that the C-terminal EF-hand (EF-2) plays a role as a trigger for Ca(2+)-induced conformational change.

Nicotinic Acid Adenine Dinucleotide Phosphate-induced Ca2+ Release: INTERACTIONS AMONG DISTINCT Ca2+ MOBILIZING MECHANISMS IN STARFISH OOCYTES

An intracellular mechanism activated by nicotinic acid adenine dinucleotide phosphate (NAADP(+)) contributes to intracellular Ca(2+) release alongside inositol 1,4,5-trisphosphate (Ins-P(3)) and ryanodine receptors. The NAADP(+)-sensitive mechanism has been shown to be operative in sea urchin eggs, ascidian eggs, and pancreatic acinar cells. Furthermore, most mammalian cell types can synthesize NAADP(+), with nicotinic acid and NADP(+) as precursors. In this contribution, NAADP(+)-induced Ca(2+) release has been investigated in starfish oocytes. Uncaging of injected NAADP(+) induced Ca(2+) mobilization in both immature oocytes and in oocytes matured by the hormone 1-methyladenine (1-MA). The role of extracellular Ca(2+) in NAADP(+)-induced Ca(2+) mobilization, which was minor in immature oocytes, was instead essential in mature oocytes. Thus, the NAADP(+)-sensitive Ca(2+) pool, which is known to be distinct from those sensitive to inositol 1,4,5-trisphosphate or cyclic ADPribose, apparently migrated closer to (or became part of) the plasma membrane during the maturation process. Inhibition of both Ins-P(3) and ryanodine receptors, but not of either alone, substantially inhibited NAADP(+)-induced Ca(2+) mobilization in both immature and mature oocytes. The data also suggest that NAADP(+)-induced Ca(2+) mobilization acted as a trigger for Ca(2+) release via Ins-P(3) and ryanodine receptors.

T-type Ca2+ current as a trigger for Ca2+ release from the sarcoplasmic reticulum in guinea-pig ventricular myocytes.

1. We have investigated whether Ca2+ entry through T-type Ca2+ channels participates in triggering Ca2+ release from the sarcoplasmic reticulum (SR) in single guinea-pig ventricular myocytes (whole-cell voltage clamp, K5fura-2 as [Ca2+]i indicator; all monovalent cations replaced by impermeant ions to record uncontaminated Ca2+ currents; T = 23 or 36 degrees C). 2. T-type Ca2+ currents were elicited from a holding potential of -90 mV during steps to -50 to -20 mV. For steps to -50 mV, very small [Ca2+]i transients could be recorded with high loading of the SR (peak Delta[Ca2+]i, 67 +/- 41 nM; n = 9). 3. For steps to -40, -30 and -20 mV, we compared the amplitude of Ca2+ release for a holding potential of -50 mV with L-type Ca2+ current only to Ca2+ release for a holding potential of -90 mV with both T- and L-type Ca2+ current. Significantly more Ca2+ release was observed with T-type current present, and both the T-type current and the additional Ca2+ release were suppressed by 50 microM NiCl2. 4. Ca2+ influx through T-type Ca2+ channels triggered less Ca2+ release than a comparable Ca2+ influx through L-type Ca2+ channels. 5. Rapid block of T-type Ca2+ current during the action potential (50 microM NiCl2 during steady-state stimulation at 1 or 2 Hz) did not immediately reduce Ca2+ release, although a small decrease was observed after longer application. 6. We conclude that T-type Ca2+ current can trigger Ca2+ release from the SR albeit less efficiently than L-type Ca2+ current. T-type current is most likely to provide only a small contribution to the trigger for Ca2+ release in normal conditions. These results support the hypothesis that L-type Ca2+ channels have a privileged role in excitation-contraction coupling.

The 1996 Merck Frosst Award. The voltage-sensitive release mechanism: a new trigger for cardiac contraction.

Contraction in mammalian heart is initiated by a rapid rise in intracellular free calcium (Ca2+) triggered by excitation of the sarcolemma. Traditional views of cardiac excitation-contraction coupling have focused on the importance of Ca(2+)-induced Ca2+ release from the sarcoplasmic reticulum as a major source for this increase in Ca2+. Influx of Ca2+, primarily through L-type Ca2+ channels and the sodium-calcium (Na(+)-Ca2+) exchanger, is considered to be the main trigger for Ca(2+)-induced Ca2+ release. However, we recently have discovered a new trigger for excitation-contraction coupling in experiments on isolated ventricular myocytes under voltage clamp conditions. This trigger is a voltage-sensitive release mechanism that initiates release of Ca2+ from the sarcoplasmic reticulum. This article reviews the development of the concept of voltage-activated Ca2+ release in heart and discusses the importance of this discovery to the physiology, pathophysiology, and pharmacology of cardiac contraction.

Agonist-mediated activation of PLA2 initiates Ca2+ mobilization in intestinal longitudinal smooth muscle.

Recent studies have shown that Ca2+ mobilization in longitudinal muscle is initiated by inositol 1,4,5-trisphosphate (IP3)-independent Ca2+ influx that acts as a trigger for Ca(2+)-induced Ca2R release. The present study examined whether arachidonic acid (AA) acts as mediator of the initial Ca2+ influx. Cholecystokinin octapeptide caused transient concentration-dependent increase in AA release in dispersed intestinal longitudinal but not circular muscle cells followed by sustained increase in both muscle cell types. The initial increase in AA release coincided with the initial Ca2+ transient and muscle contraction: all three events were abolished by guanosine 5'-O-(2-thiodiphosphate), pertussis toxin (PTX), and the phospholipase A2 (PLA2) inhibitor, dimethyleicosadienoic acid, but were not affected by calphostin C or neomycin. Exogenous AA caused concentration-dependent contraction and increase in cytosolic free Ca2+ ([Ca2+]i) in longitudinal but not circular muscle cells; both events were abolished by Ca2+ channel blockers. Depletion of Ca2+ stores with thapsigargin attenuated with thapsigargin attenuated agonist- and AA-mediated increase in [Ca2+]i and contraction in longitudinal muscle cells: the residual [Ca2+]i increase (35%) and contraction (25%) reflected the component of Ca2+ influx. We conclude that AA released by agonist-mediated G protein-dependent PTX-sensitive activation of PLA2 mediates Ca2+ influx, which then triggers Ca(2+)-induced Ca2+ release. The process is independent of phosphatidylinositol hydrolysis and occurs exclusively in longitudinal smooth muscle, in which Ca2+ release channels are highly sensitive to Ca2+, ryanodine, and cyclic ADP-ribose and insensitive to IP3.

Steroidogenesis in isolated adrenal cells: Excitation by calcium

Calcium salts were found to replace ACTH in inducing steroidogenesis in isolated adrenocortical cells from rats. This Ca-specific stimulation occurred when the cation was presented to the cells in the presence of phosphate and carbonate as counter-ions under conditions which favoured the formation of colloidal calcium. Colloid generation and stabilization was facilitated by the use of calcium buffers and gelatin. Stable soluble or sparingly soluble calcium complexes were inactive. The preparation of cells and metastable calcium solutions is described in detail. The Ca trigger was sensitive to Ca deprivation or inhibitors of Ca transport and could be replaced by Sr. The relative role of Ca and cyclic AMP as second messengers is discussed.