L-type calcium channels in cardiac and skeletal muscle. Purification and phosphorylation

Heart Block Initiated by Candlenut Ingestion

The candlenut tree is a tropical plant that has a vast number of uses which include fertilizer, dye, ink for tattooing, and fuel. The inner seed of the nut is the most utilized portion of the plant and is often sold as a food additive, natural laxative, or a weight loss supplement. Unfortunately, the seed itself is very toxic when consumed whole and in its raw state. Typical symptoms of toxicity include abdominal pain, vomiting, and diarrhea. Rarely, it can cause cardiac dysrhythmias, most commonly bradycardia and atrioventricular heart block. We present a case of a young adult female with no significant past medical history who developed typical symptoms of toxicity, as well as atrioventricular heart block following ingestion of a candlenut. Most documented cases describe complete resolution of gastrointestinal and cardiac symptoms about one week following ingestion; however, treatment while inpatient can consist of inotropes or vasopressor support, intravenous fluid hydration, electrolyte replacement, and antiemetics. Although the mechanism of action remains unclear, this report provides physicians with an understanding of the risks of ingestion and the knowledge of typical management of the toxic effects of the candlenut.

Evidence for two concentration-dependent processes for beta-subunit effects on alpha1B calcium channels

beta-Subunits of voltage-dependent Ca(2+) channels regulate both their expression and biophysical properties. We have injected a range of concentrations of beta3-cDNA into Xenopus oocytes, with a fixed concentration of alpha1B (Ca(V)2.2) cDNA, and have quantified the corresponding linear increase of beta3 protein. The concentration dependence of a number of beta3-dependent processes has been studied. First, the dependence of the a1B maximum conductance on beta3-protein occurs with a midpoint around the endogenous concentration of beta3 (approximately 17 nM). This may represent the interaction of the beta-subunit, responsible for trafficking, with the I-II linker of the nascent channel. Second, the effect of beta3-subunits on the voltage dependence of steady-state inactivation provides evidence for two channel populations, interpreted as representing alpha1B without or with a beta3-subunit, bound with a lower affinity of 120 nM. Third, the effect of beta3 on the facilitation rate of G-protein-modulated alpha1B currents during a depolarizing prepulse to +100 mV provides evidence for the same two populations, with the rapid facilitation rate being attributed to Gbetagamma dissociation from the beta-subunit-bound alpha1B channels. The data are discussed in terms of two hypotheses, either binding of two beta-subunits to the alpha1B channel or a state-dependent alteration in affinity of the channel for the beta-subunit.

Expression of alpha 1D subunit mRNA is correlated with L-type Ca2+ channel activity in single neurons of hippocampal "zipper" slices

L-type voltage-sensitive Ca(2+) channels (L-VSCCs) play an important role in developmental and aging processes, as well as during normal function of brain neurons. Here, we tested a prediction of the hypothesis that membrane density of functional L-VSCCs is regulated by the level of gene expression for its alpha(1D) pore-forming subunit. If so, alpha(1D) mRNA and L-VSCC activity should be positively correlated within individual neurons. Conventional methods of aspiration and/or acute cell dissociation used in prior single-cell studies have generally yielded variable and incomplete recovery of intracellular mRNA. Thus, quantitative relationships between channel function and expression have been difficult to define. In this study, we used the partially dissociated ("zipper") hippocampal slice preparation as a method for collecting a single neuron's mRNA complement. This preparation, developed to expose neuronal somata for recording, also enables the extraction of a neuron with major processes largely intact. Thus, single-cell measures of gene/mRNA expression can be based on approximately the cell's full set of mRNA transcripts. In adult and aged rat hippocampal zipper slices, L-VSCC activity was first recorded in CA1 neurons in cell-attached patch mode. The same neurons were then extracted and collected for semiquantitative reverse transcriptase-PCR analysis of alpha(1D) and calmodulin A (CaM) mRNA content. Across multiple single neurons, a significant, positive correlation was found between the rank orders of L-VSCC activity and of alpha(1D), but not CaM, mRNA expression. Thus, these studies support the possibility that the level of alpha(1D) gene expression regulates the density of functional L-VSCCs.

Developmental regulation of the L-type calcium channel alpha1C subunit expression in heart

We used Northern analyses, RNase protection assays and immunoblot analyses to examine the relationship among developmental age of the heart, abundance of mRNA and L-type calcium channel alpha1C subunit protein, and to establish the size of the native protein in heart. Northern analysis, RNase protection assays, and immunoblots were used to study RNA and protein from rat heart of various ages. In fetal and adult ventricles there was a predominant 8.3-kb transcript for the alpha1C subunit with no change in transcript size during development. RNase protection assays demonstrated a 2-fold increase in abundance of the DHP receptor message during postnatal development. Immunoblots identified a 240 kD protein, corresponding to the predicted molecular mass of the full length alpha1C subunit. No change in size of protein for the alpha1C subunit was observed at any developmental stage and there was no evidence for a truncated isoform. There was an approximate 2-fold increase in alpha1C subunit protein in ventricular homogenates during postnatal development. Thus, in the developing rat heart, alterations in calcium channel properties during development appear to result neither from alternative splicing that produces a smaller transcript for the alpha1C subunit nor from expression of a truncated protein, but at least in part from transcriptionally-regulated expression of the 240 kDa polypepde.

Identification of PK-A phosphorylation sites in the carboxyl terminus of L-type calcium channel alpha 1 subunits

Full length L-type calcium channel alpha 1 subunits are rapidly phosphorylated by protein kinase A (PK-A) in vitro and in vivo at sites located in their long carboxyl terminal tails. In skeletal muscle, heart, and brain the majority of biochemically isolated alpha 1 subunits lacks these phosphorylation sites due to posttranslational proteolytic processing. Truncation may therefore modify the regulation of channel activity by PK-A. We combined site-directed mutagenesis and heterologous expression to investigate the extent to which putative cAMP-dependent phosphorylation sites in the C-terminus of alpha 1 subunits from skeletal muscle, heart, and brain are phosphorylated in vitro. The full length size form of wild-type and mutant calcium channel alpha 1 subunits was obtained at high yield after heterologous expression in Saccharomyces cerevisiae. Like in fetal rabbit myotubes [Rotman, E.I., et al. (1995) J. Biol. Chem. 270, 16371-16377], the rabbit skeletal muscle alpha 1 C-terminus was phosphorylated at serine residues 1757 and 1854. In the carboxyl terminus of alpha 1S from carp skeletal muscle and alpha 1C from rabbit heart a single serine residue was phosphorylated by PK-A in vitro. The C-terminus of alpha 1D was phosphorylated at more than one site. Employing deletion mutants, most of the phosphorylation ( > 70%) was found to occur between amino acid residues 1805 and 2072. Serine 1743 was identified as additional phosphorylation site in alpha 1D. We conclude that in class S and C calcium channels the most C-terminal phosphorylation sites are substrate for PK-A in vitro, whereas in class D calcium channels phosphorylation also occurs at a site which is likely to be retained even after posttranslational truncation.

Multifunctional Ca2+/calmodulin-dependent protein kinase mediates Ca(2+)-induced enhancement of the L-type Ca2+ current in rabbit ventricular myocytes

The intracellular mechanism underlying the Ca(2+)-induced enhancement of the L-type Ca2+ current (ICa) was examined in adult rabbit cardiac ventricular myocytes by using patch-clamp methodology. Internal Ca2+ was elevated by flash photolysis of the Ca2+ chelator Nitr 5, and intracellular Ca2+ levels were simultaneously monitored by Fluo 3 fluorescence. Flash photolysis of Nitr 5 produced a rapid (< 1-second) elevation of internal Ca2+, which led to enhancement (39% to 51% above control) of the peak inward Ca2+ current after a delay of 20 to 120 seconds. Internal dialysis of myocytes with synthetic inhibitory peptides derived from the pseudosubstrate (peptide 273-302) and calmodulin binding (peptide 291-317) regions within the regulatory domain of multifunctional Ca2+/calmodulin-dependent protein kinase (CaM kinase) blocked enhancement of ICa produced by elevation of internal Ca2+ but not that produced by beta-adrenergic stimulation. These inhibitory peptides also had no effect on the elevation of internal Ca2+ produced by flash photolysis of Nitr 5. A pseudosubstrate inhibitory peptide derived from protein kinase C had no significant effect on Ca(2+)-dependent enhancement of ICa. We conclude that CaM kinase mediates the Ca(2+)-induced enhancement of ICa in mammalian cardiac myocytes by a mechanism likely involving direct phosphorylation of the L-type Ca2+ channel complex or an associated regulatory protein.

Developmental changes in the actions of phosphatase inhibitors on calcium current of rabbit heart cells

We used whole-cell voltage clamp to compare the modulation of calcium current density (ICa, picoampere per picofarad) of freshly isolated, adult and newborn rabbit heart in response to intracellular application of microcystin and okadaic acid, both of which block phosphatase activity of phosphatase type 1 and 2A. Newborn cells showed a much larger response to the intracellular application of either microcystin or okadaic acid than did adult cells. In newborn cells, the application of microcystin produced an increase in ICa which appeared to maximize ICa, as shown by the rise in ICa to levels which could be reached by application of 10 microM forskolin or by the intracellular application of 200 microM 3',5'-cyclic adenosine monophosphate (cAMP). In adult cells, the maximal response to microcystin was considerably less than that obtainable with forskolin or cAMP. After achieving a maximal response with microcystin, the addition of forskolin increased ICa further in adult cells but elicited no additional response in newborn cells. The treatment of cells with 0.1 microM isoproterenol, a concentration approximately equal to that required for a half-maximal response, strongly potentiated the effect of microcystin in newborn cells, but not in adult cells. We propose that newborn rabbit heart cells compared with adult rabbit heart cells have a greater level of protein phosphatase activity (perhaps combined with a somewhat greater kinase activity), a greater proportion of the protein phosphatase activity in the form of protein phosphatase type 1 (which is inhibited by isoproterenol) and a greater dependence on the inhibition of protein phosphatase as a mechanism of action of isoproterenol, compared with the increase in kinase activity on calcium channels.

Mechanisms for cardiac depression induced by phorbol myristate acetate in working rat hearts

1. The effects of the phorbol ester, phorbol myristate acetate (PMA) were examined on function and energy metabolism in the isolated working heart of the rat. 2. At a concentration of 10(-9) M PMA produced a rapid loss in cardiac function in terms of aortic flow rate (AFR) and coronary flow rates (CFR) whereas a similar concentration of 4 alpha-phorbol 12,13-didecanoate was ineffective. At a concentration of 10(-10) M, the PMA-induced depression was more gradual but nevertheless very pronounced with an almost total loss in AFR after 30 min perfusion. The reduction in CFR was more moderate than that observed with respect to AFR. 3. The protein kinase C (PKC) inhibitor (+/-)-1-O-hexadecyl-2-O-acylglycerol significantly attenuated the loss in AFR and CFR following addition of PMA. 4. Two inhibitors of Na+/H+ exchange, amiloride and quinacrine, totally prevented the reduction in AFR. Although the PMA-induced depression in CFR was also attenuated by both amiloride and quinacrine, these effects were not significant, probably reflecting the less pronounced effect of PMA on this parameter. 5. Nifedipine, a dihydropyridine calcium channel blocker reduced PMA toxicity to a similar degree as Na+/N+ exchange inhibition whereas the calcium channel agonist Bay K 8644 was without effect. 6. Tissue content of energy metabolites including high energy phosphates, total adenine nucleotides or lactate were not significantly affected by PMA perfusion. 7. We conclude that PKC activation is necessary for phorbol ester-induced cardiac dysfunction. The consequence of PKC stimulation includes (1) Na+/H+ exchange activation and a subsequent elevation in intracellular calcium [Ca2+]i via Na+/Ca2+ exchange and (2) PKC-dependent phosphorylation of the calcium channel, both of which would produce toxicity by elevation of [Ca21]i. Pharmacological manipulation of any of these steps prevents PMA toxicity by virtue of a reduction in the accumulation of [Ca21]i. PMA effects or their prevention are unrelated to any changes in energy metabolism.