Internal Ca Release Research Articles

The second messenger linking receptor activation to internal Ca release in liver.

The increase in cytosolic [Ca2+] induced by Ca-mobilizing hormones in liver is mainly due to release of Ca from intracellular stores. For Ca to be released from internal sites a messenger must be formed at the plasma membrane which diffuses into the cytosol to signal Ca release from the intracellular organelles. One of the first actions of these hormones is to cause breakdown of the polyphosphoinositides to form soluble inositol phosphates. Some evidence for the idea that these substances could be the second messenger has been obtained in pancreatic acinar cells. Here we have found that hormone activation of hepatocytes causes rapid breakdown of phosphatidylinositol 4,5-bisphosphate [ PtdIns (4,5)P2] to form inositol trisphosphate ( InsP3 ). When applied to permeabilized hepatocytes, InsP3 releases Ca from non-mitochondrial ATP-dependent pools. This suggests that InsP3 could be the messenger linking Ca-mobilizing receptor activation to intracellular Ca release in liver.

Inositol lipids and cell stimulation in mammalian salivary gland

The rat parotid salivary gland shows marked alterations in phospholipid metabolism when stimulated by certain agonists. These agonists are those which cause cellular Ca mobilization by activation of muscarinic, α-adrenergic or peptidergic (substance P) receptors. The phospholipid changes apparently reflect the activation of a phosphoinositide-phosphatidic acid cycle, the precise pathways of which are not known with certainty. The observed effects include (1) an increased labelling by 32PO4 of phosphatidylinositol and phosphatidic acid, (2) net synthesis of phosphatidic acid, (3) net breakdown of phosphatidylinositol and phosphatidylinositol-4,5-bisphosphate. These effects apparently do not require the presence of extracellular Ca or the release of internal Ca and cannot be produced by the artificial introduction of Ca into the cytosol with Ca ionophores. These findings are consistent with the view that a receptor-mediated alteration in phosphoinositide metabolism represents an early step in the stimulus-response pathway in the parotid acinar cell. It has been suggested that phosphatidic acid synthesis might be of central importance in mediating Ca influx and that PIP2 breakdown might play a role in activation of Ca release. Evidence for these latter ideas is for the present largely circumstantial.

Intracellular Ca release in skinned smooth muscle.

The release of internal Ca from saponin-treated skinned smooth muscle of guinea pig taenia caecum was studied. The amount of Ca released was estimated by the area under the contraction curve during treatment with 25 mM caffeine in the presence of 0.1 mM EGTA. The magnitude of the caffeine response in skinned muscle, after loading with 10(-6) M Ca for 3 min, was similar to that in the depolarized muscle in the presence of EGTA before treatment with saponin. This suggests that Ca in the skinned muscle was in a physiological range after loading. The release of Ca from the storage site could be facilitated by Ca itself when the skinned muscle was exposed to Ca above 3 x 10(-6) M. An increase in environmental MG concentration suppressed the Ca-induced Ca release mechanism. Sudden replacement of propionate with Cl in the bathing solution made it possible to release Ca from the storage site. This "depolarization"-induced Ca release occurred only immediately after the application of Cl; thereafter, the Ca release mechanism seemed to be inactivated by the prolonged presence of Cl. These results suggest that two mechanisms of Ca release operate in smooth muscle: (a) release induced by Ca itself, and (b) release by "depolarization".

Ca-movement controlling myocardial contractility. I. Voltage-, current- and time-dependence of mechanical activity under voltage clamp conditions (cat papillary muscles and trabeculae).

1. Tension development and membrane currents have been measured in cat ventricular fibres applying a double sucrose-gap voltage clamp technique. 2. A well defined mechanical threshold was found at which a considerable all-or-non response occurred. This threshold obviously coincided with the threshold potential of the fast inward current (INa). Changes in [Cae] (0.8–7.5 mM) did not separate the two thresholds. Ramp clamps of decreasing steepness shifted the threshold of bothINa and that of tension development towards less negative potentials. Ramp clamps ineffective in elicitingINa did not produce phasic tension development. This suggests that a fast inward current normally triggers the internal Ca release. 3. In rhythmically activated preparations the steady state voltage-tension relationship rose roughly in parallel toICa in a S-shaped manner up to about zero potential. With further depolarizations the steady-state tension development tended to decline again. It was concluded that this voltage dependent gain of contractile activation reflects the time integral of transmembrane Ca supply provided from the extracellular space. 4. Under steady state conditions peak tension produced by square clamps of a given suprathreshold voltage rose with prolongation of the clamp duration up to a maximum at 500–700 msec. This maximum in turn depended on the clamp potential applied. From the results obtained, a tension-time-voltage relationship has been drawn. Even under very long lasting depolarizations (up to 10 sec) usually no tonic responses occurred. 5. A muscle once activated by a suprathreshold square clamp did not respond to a second depolarization unless the membrane had been repolarized to a given level and for a given duration. The amplitude of contractile response to second clamp steps depended in a characteristic manner on both the voltage and the duration of the recovery period. 6. Mechanical transients typically observed when either the voltage or the duration of clamp steps had been changed were analyzed in order to obtain information about the kinetics of intracellular Ca movements assumed to take place in a multicompartment model derived from structural and functional observations.