scholarly journals Two types of Ca2+ currents with different sensitivities to organic Ca2+ channel antagonists in guinea pig pancreatic alpha 2 cells.

1988 ◽  
Vol 91 (2) ◽  
pp. 243-254 ◽  
Author(s):  
P Rorsman
Keyword(s):  
Steady State ◽  
Guinea Pig ◽  
Voltage Clamp ◽  
Maximum Amplitude ◽  
Time Dependent ◽  
Voltage Clamp Technique ◽  
Electrode Voltage ◽  
Steady State Inactivation ◽  
Ca2 Channels ◽  
Better Than

The possibility that guinea pig pancreatic alpha 2 cells are equipped with more than one type of Ca2+ channel was explored using the patch-electrode voltage-clamp technique. At a holding potential of -100 mV, a slowly developing (tau m approximately 5 ms at -40 mV assuming m2 kinetics) Ca2+ current appeared. This conductance first became detectable at potentials of about -60 mV and reached a maximum amplitude of 50-100 pA between -30 and -20 mV. During long depolarizations, it inactivated completely (tau h approximately 100 ms at -40 mV). Half-maximal steady state inactivation was observed at about -60 mV. A second, more rapidly developing (tau m approximately 2 ms at 0 mV) Ca2+ current was observed during pulses to -40 mV and above. It had a peak amplitude of 150-200 pA between 0 and 10 mV, was less dependent on the holding potential, and inactivated very little, even during long pulses. Both conductances were blocked by Co2+ but were unaffected by tetrodotoxin. The rapidly developing current differed from the slowly developing one in being sensitive to the antagonists D-600 and nifedipine, conducting Ba2+ better than Ca2+, increasing upon exposure to forskolin, and showing time-dependent decay (rundown). These findings indicate that the alpha 2 cells are equipped with two kinds of Ca2+ channels.

scholarly journals Voltage- and time-dependent K+ channel currents in the basolateral membrane of villus enterocytes isolated from guinea pig small intestine.

1994 ◽  
Vol 103 (3) ◽  
pp. 429-446 ◽  
Author(s):  
H Tatsuta ◽  
S Ueda ◽  
S Morishima ◽  
Y Okada
Keyword(s):  
Steady State ◽  
Guinea Pig ◽  
Time Course ◽  
Basolateral Membrane ◽  
Time Dependent ◽  
K Channel ◽  
Voltage Dependent ◽  
Steady State Inactivation ◽  
K Current ◽  
K Currents

Patch-clamp studies were carried out in villus enterocytes isolated from the guinea pig proximal small intestine. In the whole-cell mode, outward K+ currents were found to be activated by depolarizing command pulses to -45 mV. The activation followed fourth order kinetics. The time constant of K+ current activation was voltage-dependent, decreasing from approximately 3 ms at -10 mV to 1 ms at +50 mV. The K+ current inactivated during maintained depolarizations by a voltage-independent, monoexponential process with a time constant of approximately 470 ms. If the interpulse interval was shorter than 30 s, cumulative inactivation was observed upon repeated stimulations. The steady state inactivation was voltage-dependent over the voltage range from -70 to -30 mV with a half inactivation voltage of -46 mV. The steady state activation was also voltage-dependent with a half-activation voltage of -22 mV. The K+ current profiles were not affected by chelation of cytosolic Ca2+. The K+ current induced by a depolarizing pulse was suppressed by extracellular application of TEA+, Ba2+, 4-aminopyridine or quinine with half-maximal inhibitory concentrations of 8.9 mM, 4.6 mM, 86 microM and 26 microM, respectively. The inactivation time course was accelerated by quinine but decelerated by TEA+, when applied to the extracellular (but not the intracellular) solution. Extracellular (but not intracellular) applications of verapamil and nifedipine also quickened the inactivation time course with 50% effective concentrations of 3 and 17 microM, respectively. Quinine, verapamil and nifedipine shifted the steady state inactivation curve towards more negative potentials. Outward single K+ channel events with a unitary conductance of approximately 8.4 pS were observed in excised inside-out patches of the basolateral membrane, when the patch was depolarized to -40 mV. The ensemble current rapidly activated and thereafter slowly inactivated with similar time constants to those of whole-cell K+ currents. It is concluded that the basolateral membrane of guinea pig villus enterocytes has a voltage-gated, time-dependent, Ca(2+)-insensitive, small-conductance K+ channel. Quinine, verapamil, and nifedipine accelerate the inactivation time course by affecting the inactivation gate from the external side of the cell membrane.

scholarly journals Voltage-dependent inactivation of slow calcium channels in intact twitch muscle fibers of the frog.

1989 ◽  
Vol 94 (5) ◽  
pp. 937-951 ◽  
Author(s):  
G Cota ◽  
E Stefani
Keyword(s):  
Steady State ◽  
Rate Constant ◽  
Voltage Dependence ◽  
Muscle Fibers ◽  
Dependent Manner ◽  
Inactivation Curve ◽  
Voltage Dependent ◽  
Steady State Inactivation ◽  
Intact Muscle ◽  
Ca2 Channels

Inactivation of slow Ca2+ channels was studied in intact twitch skeletal muscle fibers of the frog by using the three-microelectrode voltage-clamp technique. Hypertonic sucrose solutions were used to abolish contraction. The rate constant of decay of the slow Ca2+ current (ICa) remained practically unchanged when the recording solution containing 10 mM Ca2+ was replaced by a Ca2+-buffered solution (126 mM Ca-maleate). The rate constant of decay of ICa monotonically increased with depolarization although the corresponding time integral of ICa followed a bell-shaped function. The replacement of Ca2+ by Ba2+ did not result in a slowing of the rate of decay of the inward current nor did it reduce the degree of steady-state inactivation. The voltage dependence of the steady-state inactivation curve was steeper in the presence of Ba2+. In two-pulse experiments with large conditioning depolarizations ICa inactivation remained unchanged although Ca2+ influx during the prepulse greatly decreased. Dantrolene (12 microM) increased mechanical threshold at all pulse durations tested, the effect being more prominent for short pulses. Dantrolene did not significantly modify ICa decay and the voltage dependence of inactivation. These results indicate that in intact muscle fibers Ca2+ channels inactivate in a voltage-dependent manner through a mechanism that does not require Ca2+ entry into the cell.

Stimulatory effect of ouabain on T- and L-type calcium currents in guinea pig cardiac myocytes

1990 ◽  
Vol 258 (5) ◽  
pp. H1620-H1623 ◽  
Author(s):  
B. Le Grand ◽  
E. Deroubaix ◽  
A. Coulombe ◽  
E. Coraboeuf
Keyword(s):  
Steady State ◽  
Guinea Pig ◽  
Ventricular Myocytes ◽  
Calcium Currents ◽  
High Threshold ◽  
Inactivation Curve ◽  
Cell Recording ◽  
Steady State Inactivation ◽  
Low Threshold ◽  
Free Media

The effect of 10(-6) M ouabain on macroscopic low-threshold T-type Ca2+ and high-threshold L-type Ca2+ currents was studied by whole cell recording in isolated guinea pig ventricular myocytes superfused with K-free, Na-free media, i.e., after suppression of Na-K-ATPase activity and Na influx through the Na-Ca exchanger. Under such conditions, the amplitudes of the two currents were significantly increased by ouabain. In particular, the current occurring in the -50 to -20 mV range (T-type Ca2+) was increased two- to threefold by ouabain and suppressed by 40 microM Ni2+. Ouabain shifted by approximately 10 mV toward negative potentials the steady-state inactivation curve of the T-type Ca2+ current but not that of the L-type Ca2+ current. It is concluded that ouabain enhances not only L-type Ca2+ current but also T-type Ca2+ current possibly through different mechanisms.

Ionic currents of smooth muscle cells isolated from the ctenophore Mnemiopsis

1988 ◽  
Vol 233 (1271) ◽  
pp. 99-121 ◽  
Keyword(s):  
Smooth Muscle ◽  
Steady State ◽  
Smooth Muscle Cells ◽  
Voltage Clamp ◽  
Muscle Cells ◽  
Ionic Currents ◽  
Patch Clamping ◽  
Excitable Cells ◽  
Inward Current ◽  
Electrode Voltage

The ionic currents of smooth muscle cells isolated from the ctenophore Mnemiopsis were examined by using conventional two-electrode voltage clamp and whole-cell patch clamping methods. Several separable currents were identified. These include: (1) a transient and (2) a steady-state voltage-activated inward current; both are tetrodotoxin (TTX) and saxitoxin (STX) insensitive, partly reduced by decreasing external Ca 2+ or Na + or by addition of 5 mM Co 2+ , D-600 or verapamil and are totally blocked with 5 mM Cd 2+ ; (3) an early, transient, cation-dependent, outward K + current ( I Kca/Na ); (4) a transient, voltage-activated, outward K + current provisionally identified as I A ; (5) a delayed, steady-state, voltage-activated outward K + current ( I K ) and (6) a late, transient, outward K + current which is blocked by Cd 2+ and evident only during long voltage pulses. Despite their phylogenic origin, most of these currents are similar to currents identified in many vertebrate smooth and cardiac muscle preparations, and other excitable cells in higher animals.

Leukocytic pyrogen effects on prostaglandins in hypothalamic tissue slices

1987 ◽  
Vol 253 (1) ◽  
pp. R71-R76 ◽  
Author(s):  
I. M. Scott ◽  
R. H. Fertel ◽  
J. A. Boulant
Keyword(s):  
Steady State ◽  
Guinea Pig ◽  
Dose Response ◽  
Time Dependent ◽  
Tissue Slices ◽  
Prostaglandin F2 Alpha ◽  
Leukocytic Pyrogen ◽  
Continuous Dose ◽  
Hypothalamic Tissue ◽  
Steady State Levels

Some studies suggest that leukocytic pyrogen (LP) increase hypothalamic prostaglandins which, in turn, affect hypothalamic thermoregulatory neurons to produce fever. The present study used radioimmunoassays to quantitate the ability of guinea pig hypothalamic tissue slices to produce prostaglandin E2 (PGE2), prostaglandin F2 alpha (PGF2 alpha), 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), and thromboxane B2 (TxB2). Dose- and time-dependent prostaglandin increases occurred when these slices were perfused with LP media. Steady-state levels of tissue release were reached at 0-3 min for 6-keto-PGF1 alpha, at 6-9 min for PGE2 and PGF2 alpha, and at 12-15 min for TxB2. With the exception of 6-keto-PGF1 alpha, all substances showed continuous dose-response relationships for concentrations ranging from 0.001 to 0.25 LP dilutions. Tissue PGE2, for example, was 0.7 pg X min-1 X mg-1 with the 0.001 LP dilution and 8.7 pg X min-1 X mg-1 with the 0.25 LP dilution. Indomethacin blocked much of the LP-induced prostaglandin increase. Although there is a relationship between hypothalamic LP and prostaglandins in response to physiological LP levels, tissue prostaglandins are several orders of magnitude lower than concentrations necessary to produce fever by hypothalamic microinjection. This suggests that prostanoids, such as PGE2, may not be the sole mediators of fever induced by leukocytic pyrogen.

scholarly journals Inactivation of excitation-contraction coupling in rat extensor digitorum longus and soleus muscles.

1988 ◽  
Vol 91 (5) ◽  
pp. 737-757 ◽  
Author(s):  
M Chua ◽  
A F Dulhunty
Keyword(s):  
Steady State ◽  
Membrane Potential ◽  
Voltage Clamp ◽  
Extensor Digitorum Longus ◽  
Threshold Concentration ◽  
Extensor Digitorum ◽  
Excitation Contraction Coupling ◽  
Contraction Coupling ◽  
Steady State Inactivation ◽  
Spontaneous Relaxation

K contractures and two-microelectrode voltage-clamp techniques were used to measure inactivation of excitation-contraction coupling in small bundles of fibers from rat extensor digitorum longus (e.d.l.) and soleus muscles at 21 degrees C. The rate of spontaneous relaxation was faster in e.d.l. fibers: the time for 120 mM K contractures to decay to 50% of maximum tension was 9.8 +/- 0.5 s (mean +/- SEM) in e.d.l. and 16.8 +/- 1.7 s in soleus. The rate of decay depended on membrane potential: in e.d.l., the 50% decay time was 14.3 +/- 0.7 s for contractures in 80 mM K (Vm = 25 mV) and 4.9 +/- 0.4 s in 160 mM K (Vm = -3 mV). In contrast to activation, which occurred with less depolarization in soleus fibers, steady state inactivation required more depolarization: after 3 min at -40 mV in 40 mM K, the 200 mM K contracture amplitude in e.d.l. fell to 28 +/- 10% (n = 5) of control, but remained at 85 +/- 2% (n = 6) of control in soleus. These different inactivation properties in e.d.l. and soleus fibers were not influenced by the fact that the 200 mM K solution used to test for steady state inactivation produced contractures that were maximal in soleus fibers but submaximal in e.d.l.: a relatively similar depression was recorded in maximal (200 mM K) and submaximal (60 and 80 mM K) contracture tension. A steady state "pedestal" of tension was observed with maintained depolarization after K contracture relaxation and was larger in soleus than in e.d.l. fibers. The pedestal tension was attributed to the overlap between the activation and inactivation curves for tension vs. membrane potential, which was greater in soleus than in e.d.l. fibers. The K contracture results were confirmed with the two-microelectrode voltage clamp: the contraction threshold increased to more positive potentials at holding potentials of -50 mV in e.d.l. or -40 mV in soleus. At holding potentials of -30 mV in e.d.l. or 0 mV in soleus, contraction could not be evoked by 15-ms pulses to +20 mV. Both K contracture and voltage-clamp experiments revealed that activation in soleus fibers occurred with a smaller transient depolarization and was maintained with greater steady state depolarization than in e.d.l. fibers. The K contracture and voltage-clamp results are described by a model in which contraction depends on the formation of a threshold concentration of activator from a voltage-sensitive molecule that can exist in the precursor, activator, or inactive states.

Effects of general anesthetics on current flow across membranes in guinea pig myocytes

1989 ◽  
Vol 256 (2) ◽  
pp. C273-C281 ◽  
Author(s):  
E. Niggli ◽  
A. Rudisuli ◽  
P. Maurer ◽  
R. Weingart
Keyword(s):  
Guinea Pig ◽  
Voltage Clamp ◽  
Current Flow ◽  
Optical Measurements ◽  
General Anesthetics ◽  
Voltage Range ◽  
Inward Current ◽  
Electrical Uncoupling ◽  
Steady State Inactivation ◽  
Current Systems

Myocytes were isolated from adult guinea pig ventricles. Whole cell, tight-seal recording was employed to investigate the electrical properties of the junctional (nexal membrane) and nonjunctional membrane (sarcolemma) under the influence of n-alkanols (heptanol, octanol) and halothane. Studies of cell pairs with a double voltage-clamp approach showed that these agents give rise to a reversible electrical uncoupling. Examination of single myocytes with a single voltage-clamp method showed that these substances modify several sarcolemmal current systems. The slope conductance was reduced over the entire voltage range examined (-90 to +50 mV). The Ca2+ inward current (Isi) showed a decreased amplitude and an accelerated inactivation. The repriming of Isi remained unchanged. The steady-state inactivation of Isi was shifted by 2-3 mV toward more negative potentials. Optical measurements demonstrated an increase in sarcomere spacing at rest and a decrease during peak systolic shortening. The results suggest that n-alkanols and halothane exert their effects on membrane currents via incorporation into the lipid bilayer.

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