Cardiac calcium currents at the level of single channels

Voltage-gated calcium channel antagonists and traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of death and disability in the United States. Despite more than 30 years of research, no pharmacological agents have been identified that improve neurological function following TBI. However, several lines of research described in this review provide support for further development of voltage gated calcium channel (VGCC) antagonists as potential therapeutic agents. Following TBI, neurons and astrocytes experience a rapid and sometimes enduring increase in intracellular calcium ([Ca²⁺]_i). These fluxes in [Ca²⁺]_i drive not only apoptotic and necrotic cell death, but also can lead to long-term cell dysfunction in surviving cells. In a limited number of in vitro experiments, both L-type and N-type VGCC antagonists successfully reduced calcium loads as well as neuronal and astrocytic cell death following mechanical injury. In rodent models of TBI, administration of VGCC antagonists reduced cell death and improved cognitive function. It is clear that there is a critical need to find effective therapeutics and rational drug delivery strategies for the management and treatment of TBI, and we believe that further investigation of VGCC antagonists should be pursued before ruling out the possibility of successful translation to the clinic.

Dihydropyridine block of voltage-dependent K+ currents in rat dorsal root ganglion neurons

The dihydropyridines nifedipine, nimodipine and Bay K 8644 are widely used as pharmacological tools to assess the contribution of L-type voltage-gated Ca(2+) channels to a variety of neuronal processes including synaptic transmission, excitability and second messenger signaling. These compounds are still used in neuronal preparations despite evidence from cardiac tissue and heterologous expression systems that they block several voltage-dependent K(+) (Kv) channels. Both because these compounds have been used to assess the relative contribution of L-type Ca(2+) channels to several different processes in dorsal root ganglion (DRG) neurons and because a relatively wide variety of Kv channels present in other neuronal populations is present in DRG neurons, we determined the extent to which dihydropyridines block Kv currents in these neurons. Standard whole cell patch clamp techniques were used to study acutely disassociated adult rat DRG neurons. All three dihydropyridines tested blocked Kv currents in DRG neurons; IC(50) values (concentration resulting in an inhibition that is 50% of maximum) for nifedipine and nimodipine-induced block of sustained Kv currents were 14.5 and 6.6 microM, respectively. The magnitude of sustained current block was 44+/-1.6%, 60+/-2%, and 56+/-2.9% with 10 microM nifedipine, nimodipine and Bay K 8644, respectively. Current block was occluded by neither 4-aminopyridine (5 mM) nor tetraethylammonium (135 mM). Dihydropyridine-induced block of Kv currents was not associated with a shift in the voltage-dependence of current activation or inactivation, the recovery from inactivation, or voltage dependent block. However, there was a small use-dependence to the dihydropyridine-induced block. Our results suggest that several types of Kv channels in DRG neurons are blocked by mechanisms distinct from those underlying block of Kv channels in cardiac myocytes. Importantly, our results suggest that if investigators wish to explore the contribution of L-type Ca(2+) channels to neuronal function, they should consider alternative strategies for the manipulation of these channels than the use of dihydropyridines.

Adhesion forces measured between a calcium blocker drug and its receptor in living cells using atomic force microscope

The adhesion force between the tip of an atomic force microscope cantilever derivatized with nimodipine (a calcium blocker, from the dihydropyridine class, currently used in clinical medicine for hypertension) and living cells of Saccharomyces cerevisiae (unicellular eukaryotes which portray ultrastructural features characteristic of higher eukaryotic cells) was measured. This methodology allowed us to locate (and visualize) pores on the cell surface which may be responsible for calcium transportation in the living cells. The interaction of the cantilever derivatized with the calcium blocker and a pore, which can be a calcium channel, is more intense than a non-derivatized cantilever and the pore. Outside the pore (on the rest of cell surface), a derivatized or a non-derivatized cantilever has the same pattern of adhesion force. The information obtained with this method is very important for the design of new, more potent and less toxic drugs for pharmacological use.

Comparison of the Ca2 + currents induced by expression of three cloned alpha1 subunits, alpha1G, alpha1H and alpha1I, of low-voltage-activated T-type Ca2 + channels

Expression of rat alpha1G, human alpha1H and rat alpha1I subunits of voltage-activated Ca2 + channels in HEK-293 cells yields robust Ca2 + inward currents with 1.25 mM Ca2 + as the charge carrier. Both similarities and marked differences are found between their biophysical properties. Currents induced by expression of alpha1G show the fastest activation and inactivation kinetics. The alpha1H and alpha1I currents activate and inactivate up to 1.5- and 5-fold slower, respectively. No differences in the voltage dependence of steady state inactivation are detected. Currents induced by expression of alpha1G and alpha1H deactivate with time constants of up to 6 ms at a test potential of - 80 mV, but currents induced by alpha1I deactivate about three-fold faster. Recovery from short-term inactivation is more than three-fold slower for currents induced by alpha1H and alpha1I in comparison to alpha1G. In contrast to these characteristics, reactivation after long-term inactivation was fastest for currents arising from expression of alpha1I and slowest in cells expressing alpha1H calcium channels. The calcium inward current induced by expression of alpha1I is increased by positive prepulses while currents induced by alpha1H and alpha1G show little ( < 5%) or no facilitation. The data thus provide a characteristic fingerprint of each channel's activity, which may allow correlation of the alpha1G, alpha1H and alpha1I induced currents with their in vivo counterparts.

Ca2+ entry through L-type voltage-sensitive Ca2+ channels stimulates the release of human chorionic gonadotrophin and placental lactogen by placental explants

The release of human chorionic gonadotrophin (hCG) and placental lactogen (hPL) by human placental explants can be stimulated by Ca2+ entry. The aim of the present study was to characterize the modality of Ca2+ entry in the presence of high extracellular K+ concentration ([K+]o). A rise in [K+]o from 5 to > or = 50 mM induced a rapid and marked increase in the release of hCG and hPL from human term placental explants. The stimulatory effects of an excess [K+]o on the release of hCG and hPL were blocked in the absence of extracellular Ca2+ or in the presence of 0.5 mM Co2+. The presence of 50 microM methoxyverapamil, 20 microM nifedipine or 40 microM Cd2+ in the medium inhibited the stimulatory effects of [K+]o addition. Lastly, 40 microM Ni2+ failed to affect the increases in hCG and hPL releases elicited by [K+]o addition. Our data clearly show that a rise in [K+]o stimulates the release of hCG and hPL from placental explants. These secretory effects can be viewed as resulting from a Ca2+ entry through voltage-sensitive Ca2+ channels of the L-type.

Erythromycin stimulates ileal motility by activation of dihydropyridine-sensitive calcium channels

Erythromycin, a macrolide antibiotic, is a potent stimulant of small bowel motor activity (MA) which may motility either via the peptide motilin receptor or neural mechanisms. We hypothesized that erythromycin stimulates directly stimulates smooth muscle cells by a calcium-mediated event. Thus, we evaluated the effect of neuronal blockade with tetrodotoxin, muscarinic blockade with atropine, and opiate blockade with naloxone on erythromycin-stimulated MA in isolated perfused segments of rabbit terminal ileum. We also tested the effect of nonspecific calcium channel blockade (verapamil and cadmiun) and specific blockade (dihydroxypyridine and nichol) on erythromycin-stimulated MA. MA was measured with a multichannel continuous perfusion manometry catheter. Erythromycin caused a concentration-dependent increase in MA (ED100 5 x 10(-4) M). Tetrodotoxin, atropine, and naloxone did not effect erythromycin-stimulated MA (P greater than 0.05). Both verapamil (10(-7) M) and cadmium (10(-2)-10(-4) M) inhibited erythromycin-stimulated MA. Selective blockade of "l" type calcium channels using dihydropyridine (10(-6) M) and "t" channels with nickel (10(-2)-10(-4) M) both reversed erythromycin-stimulated MA. Since the isolated segments of terminal ileum were free of exogenous humoral and neural effects, these studies indicated that erythromycin directly stimulated MA in the terminal ileum. Furthermore, since tetrodotoxin, atropine, and naloxone did not inhibit this increase in MA, erythromycin acted by a mechanism which was independent of the intrinsic nervous and opiate systems. In conclusion, these data are consistent with the model that erythromycin stimulates ileal motility by a mechanism involving activation of dihydroxypyridine and nickel-sensitive calcium channels.

Sensitivity to Cd2+ but resistance to Ni2+ of Ca2+ inflow into rat pancreatic islets

The presence of different types [long lasting (L) and transient (T)] of active voltage-operated Ca2+ channels in islet cells was investigated by comparing the effects of Cd2+, Ni2+, and 1,4-dihydropyridines on 45Ca uptake, 45Ca efflux, and insulin release in intact rat pancreatic islets. In several other excitable cells the L-channel has been shown to be modulated by 1,4-dihydropyridines and Cd2+, whereas the T-channel was reported to be sensitive to Ni2+. Nifedipine and Cd2+ inhibited whereas BAY K 8644 enhanced the glucose (11.1, 22.2 mM)-stimulated short-term 45Ca uptake, 45Ca efflux, and insulin release. In contrast, the stimulatory effects of glucose (11.1, 22.2 mM) on 45Ca uptake, 45Ca efflux, and insulin release were unaffected by Ni2+. These findings confirm that glucose provokes Ca2+ entry mainly by activating voltage-sensitive Ca2+ channels of the L-type and suggest that the B-cell plasma membrane is not equipped with active T-type Ca2+ channels.