Effects of calcium channel antagonists on the phosphorylation of major protein kinase C substrates in the rat hippocampus

Regulation of extracellular calcium-activated cation currents by cAMP in parathyroid cells

Parathyroid cells express Ca2+-sensing receptors that couple changes in the extracellular Ca2+ concentration ([Ca2+]o) to increases in the intracellular free Ca2+ concentration ([Ca2+]i) and to the suppression of parathyroid hormone secretion. Using whole cell patch clamping, we previously identified voltage-independent Ca2+-conducting currents in bovine parathyroid cells that increased with rising [Ca2+]o and were blocked by Cd2+ and nifedipine. Because cAMP-dependent phosphorylation regulates dihydropyridine-sensitive Ca2+ channels in other systems, we tested whether cAMP modulates these currents. At 0.7 mM Ca2+, nonselective Ca2+-conducting currents were suppressed by 30-50% when the recording pipette was perfused with cAMP. High-[Ca2+]o-induced increases in membrane currents were also abrogated. The effects of cAMP were reversible and dose dependent (3 x 10(-9) to 3 x 10(-3) M) and required ATP in the pipette solution. Perfusion of the cell interior with the catalytic subunit of protein kinase A mimicked the effects of cAMP, as did perfusion of the bath with the adenylate cyclase activator forskolin. These findings support the idea that cAMP-dependent phosphorylation suppresses high-[Ca2+]o-induced cation currents and may play a role in regulating ion fluxes in parathyroid cells.

The activation of calcium/calmodulin-dependent protein kinase II after glutamate or potassium stimulation in hippocampal slices

Two forms of long-term potentiation (LTP), N-methyl-D-aspartate receptor (NMDAR) dependent and non-NMDAR dependent, have been reported in hippocampal CA1 and CA3, respectively. The present study examined the activation of CaM-kinase II (calcium/calmodulin-dependent protein kinase II) in CA1 and CA3 areas after glutamate or potassium stimulation. Rat hippocampal slices were preincubated with one of the drugs (EGTA, DL-APV, CNQX, AP3, nitrendipine, KN-62, staurosporin, and H-89) before they were stimulated with either glutamate/glycine (100 microM/1 microM) or KCl (60 mM). Hippocampal CA1 area and CA3 area were then dissected and CaM-kinase II activities were assayed in vitro. Glutamate and KCl stimulations enhanced the percentage of Ca(2+)-independent CaM-kinase II activity in CA1 area. This enhancement was suppressed by EGTA, DL-APV, CNQX, or KN-62, suggesting that the neuronal stimulation effect in CA1 area was mediating through NMDA receptors. Conversely, there was no significant enhancement of CaM-kinase II activity in the CA3 area after glutamate or KCl stimulation. Nevertheless, the percentage of calcium-independent CaM-kinase II activity in the CA3 area was suppressed by EGTA, nitrendipine, KN-62, staurosporin, or H-89, indicating that the activity of CaM-kinase II in the CA3 area was independent of NMDA receptor activation.

Regulation of Ca(2+)-conducting currents in parathyroid cells by extracellular Ca(2+) and channel blockers

High extracellular Ca2+ concentrations ([Ca2+]o) produce sustained intracellular Ca2+ responses in parathyroid cells that correlate with suppression of parathyroid hormone release. Using whole cell patch clamping, we identified two types of Ca(2+)-conducting currents in these cells. Type 1 currents were enhanced by raising [Ca2+]o and blocked by Cd2+ and nifedipine, whereas type 2 currents were resistant to blockade by these agents. Both types of membrane currents were cation nonselective, voltage independent over a broad range of membrane potentials, and blocked by the trivalent ions La3+ and Gd3+ (> 98%). Cd2+, La3+, and Gd3+ had biphasic effects on membrane conductance (Gm). At submicromolar concentrations, these ions increased Gm, whereas at higher concentrations they reduced Gm. In contrast to ionic channel blockers, nifedipine had only an inhibitory effect on the Ca(2+)-conducting currents that were sensitive to changes in [Ca2+]o (dose inhibiting 50% of maximal response = approximately 3-10 x 10(-8) M). Microfluorimetric ratio-imaging analysis of single parathyroid cells loaded with fura 2 showed that Gd3+ inhibited sustained intracellular Ca2+ responses to high [Ca2+]o. These findings suggest that the Ca(2+)-conducting currents identified in these studies may play a role in regulating intracellular Ca2+ responses in this system.

1,25-Dihydroxyvitamin D3 and 12-O-tetradecanoyl phorbol 13-acetate cause differential activation of Ca(2+)-dependent and Ca(2+)-independent isoforms of protein kinase C in rat colonocytes

Considerable evidence that alterations in protein kinase C (PKC) are intimately involved in important physiologic and pathologic processes in many cells, including colonic epithelial cells, has accumulated. In this regard, phorbol esters, a class of potent PKC activators, have been found to induce a number of cellular events in normal or transformed colonocytes. In addition, our laboratory has demonstrated that the major active metabolite of vitamin D3, 1,25(OH)2D3, also rapidly (seconds-minutes) activated PKC and increased intracellular calcium in isolated rat colonocytes. These acute responses, however, were lost in vitamin D deficiency and partially restored with the in vivo repletion of 1,25(OH)2D3. The Ca(2+)-independent or novel isoforms of PKC expressed in the rat colon and the isoform-specific responses of PKC to acute treatment with phorbol esters or 1,25(OH)2D3 have not been previously characterized. Moreover, the effects of vitamin D status on PKC isoform expression, distribution, and response to agonists are also unknown. In the present experiments, in addition to PKC-alpha, rat colonocytes were found to express the novel isoforms delta, epsilon, and zeta by Western blotting using isoform-specific PKC antibodies. The tumor-promoting phorbol ester, 12-O-tetradecanoyl phorbol 13-acetate, caused time- and concentration-dependent translocations of all these isoforms except PKC-zeta. In vitamin D deficiency, there were no alterations in colonic PKC isoform expression but significant changes in the subcellular distribution of PKC-alpha, -delta, and -zeta. Acute treatment of colonocytes from D-sufficient, but not D-deficient, rats with 1,25(OH)2D3 caused a rapid transient redistribution of only PKC-alpha from the soluble to the particulate fraction. The alterations in PKC isoform distribution and PKC-alpha responsiveness to 1,25(OH)2D3 in vitamin D deficiency were partially, but significantly, restored with 5-7 d in vivo repletion of this secosteroid. Both 12-O-tetradecanoyl phorbol 13-acetate and 1,25(OH)2D3 activated endogenous PKC, as assessed by inhibition of myristoylated alanine-rich C kinase substrate back-phosphorylation by exogenous PKC. These studies indicate that PKC-alpha, -delta, and/or -epsilon likely mediate important phorbol ester-stimulated events described in the rat colon. In contrast, PKC-alpha is implicated in the rapid (s-min) PKC-dependent events initiated by 1,25(OH)2D3 in rat colonocytes.

Phosphorylation of synapsin I and MARCKS in nerve terminals is mediated by Ca2+ entry via an Aga-GI sensitive Ca2+ channel which is coupled to glutamate exocytosis

Ca2+ entry is a prerequisite for both exocytosis and the phosphorylation of synapsin I and MARCKS proteins in mammalian cerebrocortical synaptosomes. The novel spider toxin Aga-GI completely blocks KCl-evoked glutamate exocytosis but only partially inhibits KCl-evoked cytoplasmic Ca2+ elevations, thus revealing at least two pathways for KCl-induced Ca2+ entry. Aga-GI completely attenuates KCl-induced phosphorylation of synapsin I and MARCKS proteins. We therefore conclude that both exocytosis and the phosphorylation of synapsin I and MARCKS proteins are specifically coupled to Ca2+ entry via a subset of voltage dependent Ca2+ channels at the nerve terminal which are sensitive to Aga-GI.