A calcium-readmission response recorded from Nauphoeta salivary gland acinar cells

Nanojunctions of the Sarcoplasmic Reticulum Deliver Site- and Function-Specific Calcium Signaling in Vascular Smooth Muscles

Vasoactive agents may induce myocyte contraction, dilation, and the switch from a contractile to a migratory-proliferative phenotype(s), which requires changes in gene expression. These processes are directed, in part, by Ca²⁺ signals, but how different Ca²⁺ signals are generated to select each function is enigmatic. We have previously proposed that the strategic positioning of Ca²⁺ pumps and release channels at membrane-membrane junctions of the sarcoplasmic reticulum (SR) demarcates cytoplasmic nanodomains, within which site- and function-specific Ca²⁺ signals arise. This chapter will describe how nanojunctions of the SR may: (1) define cytoplasmic nanospaces about the plasma membrane, mitochondria, contractile myofilaments, lysosomes, and the nucleus; (2) provide for functional segregation by restricting passive diffusion and by coordinating active ion transfer within a given nanospace via resident Ca²⁺ pumps and release channels; (3) select for contraction, relaxation, and/or changes in gene expression; and (4) facilitate the switch in myocyte phenotype through junctional reorganization. This should serve to highlight the need for further exploration of cellular nanojunctions and the mechanisms by which they operate, that will undoubtedly open up new therapeutic horizons.

From contraction to gene expression: nanojunctions of the sarco/endoplasmic reticulum deliver site- and function-specific calcium signals

Calcium signals determine, for example, smooth muscle contraction and changes in gene expression. How calcium signals select for these processes is enigmatic. We build on the "panjunctional sarcoplasmic reticulum" hypothesis, describing our view that different calcium pumps and release channels, with different kinetics and affinities for calcium, are strategically positioned within nanojunctions of the SR and help demarcate their respective cytoplasmic nanodomains. SERCA2b and RyR1 are preferentially targeted to the sarcoplasmic reticulum (SR) proximal to the plasma membrane (PM), i.e., to the superficial buffer barrier formed by PM-SR nanojunctions, and support vasodilation. In marked contrast, SERCA2a may be entirely restricted to the deep, perinuclear SR and may supply calcium to this sub-compartment in support of vasoconstriction. RyR3 is also preferentially targeted to the perinuclear SR, where its clusters associate with lysosome-SR nanojunctions. The distribution of RyR2 is more widespread and extends from this region to the wider cell. Therefore, perinuclear RyR3s most likely support the initiation of global calcium waves at L-SR junctions, which subsequently propagate by calcium-induced calcium release via RyR2 in order to elicit contraction. Data also suggest that unique SERCA and RyR are preferentially targeted to invaginations of the nuclear membrane. Site- and function-specific calcium signals may thus arise to modulate stimulus-response coupling and transcriptional cascades.

The aminergic control of cockroach salivary glands

The acinar salivary glands of cockroaches receive a dual innervation from the subesophageal ganglion and the stomatogastric nervous system. Acinar cells are surrounded by a plexus of dopaminergic and serotonergic varicose fibers. In addition, serotonergic terminals lie deep in the extracellular spaces between acinar cells. Excitation-secretion coupling in cockroach salivary glands is stimulated by both dopamine and serotonin. These monoamines cause increases in the intracellular concentrations of cAMP and Ca(2+). Stimulation of the glands by serotonin results in the production of a protein-rich saliva, whereas stimulation by dopamine results in saliva that is protein-free. Thus, two elementary secretory processes, namely electrolyte/water secretion and protein secretion, are triggered by different aminergic transmitters. Because of its simplicity and experimental accessibility, cockroach salivary glands have been used extensively as a model system to study the cellular actions of biogenic amines and to examine the pharmacological properties of biogenic amine receptors. In this review, we summarize current knowledge concerning the aminergic control of cockroach salivary glands and discuss our efforts to characterize Periplaneta biogenic amine receptors molecularly.

Cobalt activates potassium conductance in the plasma membrane of cultured renal epithelioid (MDCK)-cells

Cobalt has been shown to stimulate sodium transport across the distal nephron of the newt kidney. The mechanism of this action remained elusive. The present study has been performed to test for effects of cobalt on electrical properties of cultured subconfluent kidney (MDCK)-cells: cobalt (10 microM) leads to a rapid, sustained and reversible hyperpolarization of the cell membrane, paralleled by an increase of the potassium selectivity and a decrease of the resistance. Thus, cobalt increases the potassium conductance of the cell membrane. The half-maximal effect is elicited by approx. 1 microM. At extracellular calcium concentration reduced to less than 0.1 microM, cobalt (10 microM) leads to a transient hyperpolarization, which can be elicited only once. Thus, cobalt enhances the potassium conductance in a calcium dependent way. At higher concentrations (100 microM) cobalt hyperpolarizes the cell membrane only transiently even in the presence of extracellular calcium. Furthermore 100 microM cobalt interferes with ATP-induced hyperpolarization, which is known to result from calcium mediated activation of K+ channels. Thus, 100 microM cobalt may inhibit ATP-stimulated calcium entry into the cell.

Dependence of intracellular effects of GTP gamma S and inositoltrisphosphate on cell membrane potential and on external Ca ions

Single cells from rat lacrimal glands were studied with the tight-seal whole-cell recording method. Ca-dependent K and Cl currents were measured in response to dialysis with inositoltrisphosphate or GTP gamma S, two compounds known for elevating internal Ca2+ concentration. The activation of the Ca-dependent currents elicited by either compound was partially inhibited by sustained depolarization or by removal of external Ca2+. Conversely, hyperpolarization or removal of external Mg2+ led to augmentations of the Ca-dependent currents. These effects became apparent about 1 min after initiation of cell dialysis with inositoltrisphosphate- or GTP gamma S-containing solutions, and they further developed during the ensuing 10 min. Holding potential and external divalent cations did not affect the Ca-dependent currents elicited by dialysing the cells with strongly buffered solutions containing 0.5 microM free Ca2+. In Ca2+-free external saline, cell currents were independent of the holding potential. It is suggested that InsP3 augments intracellular Ca2+ levels not only by releasing Ca2+ from internal stores but also by slowly increasing the Ca permeability of the plasma membrane. The results indicate that Ca2+ entry through the plasma membrane increases with hyperpolarization. The similarity of the effects seen in InsP3- and GTP gamma S-dialysed cells gives support to the hypothesis that production of the former depends on a GTP-binding protein.

Calcium redistribution, calcification and stone formation in the parotid gland during experimental stimulation and hypercalcaemia. Cytochemical and X-ray microanalytical investigations

Distribution and redistribution of intra- and pericellular calcium was investigated in the parotid gland of rats under secretory stimulation and hypercalcaemia. The effects of hypercalcaemia and secretory stimulation and of the combination of both were compared. Calcium content was determined by atomic absorption spectrometry. Calcium distribution within the tissue was demonstrated by light microscopical ( GBHA ) staining and electron microscopical (pyroantimonate method) cytochemistry in combination with X-ray microanalysis. Typical calcium depot sites were the basal and cellular membranes, the calcium buffer organelles (i.e. mitochondria) the secretory granules and the acinar lumina. After stimulation (by isoprenalin ) a decrease of calcium-enriched secretory granules and a depletion of intracellular calcium buffer organelles occurred. During hypercalcaemia (induced by dihydrotachysterol), a calcium overloading of the cell membrane and intracellular buffer organelles without calcification was observed. Combined stimulation and hypercalcaemia induced an excessive calcium overloading of all intra- and extracellular calcium depots with excessive calcium release into the acinar lumina resulting in calcium phosphate aggregates and stone formation. Secretory stimulation and simultaneous hypercalcaemia exert potentiating effects on intracellular and intraluminal calcification proposing an importance for pathogenesis of human sialolithiasis.

The dependence of fluid secretion by mandibular salivary gland and pancreas on extracellular calcium

Acetylcholine-stimulated fluid secretion from the perfused rabbit mandibular salivary gland was inhibited in a biphasic manner when extracellular calcium concentration was reduced in the range 5 X 10(-4) - 10(-5)M. An initial rapid inhibition was followed by partial recovery to a plateau, the level of which depended upon the calcium concentration. Since no recovery was observed during substitution of calcium by strontium, recovery may depend upon an increased membrane permeability to calcium. It is concluded that acetylcholine evokes fluid secretion in this gland by enhancing calcium entry from the extracellular space, an action which can be mimicked by the calcium ionophore A23187. Changes in the electrolyte composition of saliva during calcium-depletion were such as to suggest that ductal reabsorption of sodium and chloride, and secretion of potassium are inhibited as extracellular calcium concentration is reduced. Secretin-stimulated fluid secretion from the cat pancreas was unaffected when perfusate calcium concentration was reduced to 2.5 X 10(-6)M and carbachol-stimulated amylase secretion was only slightly reduced. Since the latter is a calcium-dependent process, the source of calcium is presumably intracellular. In both glands, reducing calcium to 1 X 10(-6)M caused rapid and irreversible inhibition of fluid secretion.