Biomedical subjects

K Bokvist

Publications and source records attributed to K Bokvist.

48 records · Page 3Linked to original sources

PubMed1991

Separate processes mediate nucleotide-induced inhibition and stimulation of the ATP-regulated K(+)-channels in mouse pancreatic beta-cells.

The mechanisms by which nucleotides stimulate the activity of the ATP-regulated K(+)-channel (KATP-channel) were investigated using inside-out patches from mouse pancreatic beta-cells. ATP produces a concentration-dependent inhibition of channel activity with a Ki of 18 microns. The inhibitory action of ATP was counteracted by ADP (0.1 mM) and GDP (0.2 mM) but not GTP (1 mM). Stimulation of channel activity was also observed when ADP, GDP and GTP were applied in the absence of ATP. The ability of ADP and GDP to reactivate KATP-channels blocked by ATP declined with time following patch excision and after 30-60 min these nucleotides were without effect. During the same time period the ability of ADP and GTP to stimulate the channel in the absence of ATP was lost. In fact, ADP now blocked channel activity with 50% inhibition being observed at approximately 0.1 mM. By contrast, GDP remained a stimulator in the absence of ATP even when its ability to evoke channel activity in the presence of ATP was lost. These observations show that nucleotide-induced activation of the KATP-channel does not involve competition with ATP for a common inhibitory site but involves other processes. The data are consistent with the idea that nucleotides modulate KATP-channel activity by a number of different mechanisms that may include both regulation of cytosolic constituents and direct interaction with the channel and associated control proteins.

Adenosine Diphosphate↗

PubMed1991

Activation by adrenaline of a low-conductance G protein-dependent K+ channel in mouse pancreatic B cells.

Insulin is produced and secreted by the B cells in the endocrine pancreas. In vivo, insulin secretion is under the control of a number of metabolic, neural and hormonal substances. It is now clear that stimulation of insulin release by fuel secretagogues, such as glucose, involves the closure of K+ channels that are sensitive to the intracellular ATP concentration (KATP channels). This leads to membrane depolarization and the generation of Ca2(+)-dependent action potentials. The mechanisms whereby hormones and neurotransmitters such as adrenaline, galanin and somatostatin, which are released by intraislet nerve endings and the pancreatic D cells, produce inhibition of insulin secretion are not clear. Here we show that adrenaline suppresses B-cell electrical activity (and thus insulin secretion) by a G protein-dependent mechanism, which culminates in the activation of a sulphonylurea-insensitive low-conductance K+ channel distinct from the KATP channel.

Animals↗

PubMed1991

Alpha 2-adrenoreceptor stimulation does not inhibit L-type calcium channels in mouse pancreatic beta-cells.

The effects of alpha 2-adrenergic stimulation on the Ca(2+)-current in mouse pancreatic beta-cells were investigated using the patch-clamp technique. When using the conventional whole-cell recording configuration (dialysis of cell interior with pipette solution), addition of adrenaline (1 microM) or the alpha 2-adrenergic agonist clonidine (5 microM) failed to reduce the Ca(2+)-current, irrespective of whether intracellular GTP (or GTP gamma S) was present or not and at both physiological (1.3 mM) and elevated (10.2 mM) Ca(2+)-concentrations. In fact, in the absence of added guanine nucleotides, the agonists tended to increase the Ca(2+)-current amplitude in the presence of the higher Ca(2+)-concentration. Ca(2+)-channel activation measured at 1.3 mM Ca2+ was not affected by clonidine. Half-maximal activation was observed at approximately -20 mV. In addition, when Ca(2+)-currents were recorded from intact beta-cells, using the perforated patch whole-cell configuration, clonidine (1 microM) also failed to detectably affect the Ca(2+)-current. It is therefore suggested that the inhibition of beta-cell electrical activity and insulin-secretion produced by alpha 2-adrenoreceptor stimulation does not result from suppression of the L-type Ca(2+)-current.

Action Potentials↗

PubMed1990

Delayed rectifying and calcium-activated K+ channels and their significance for action potential repolarization in mouse pancreatic beta-cells.

The contribution of Ca2(+)-activated and delayed rectifying K+ channels to the voltage-dependent outward current involved in spike repolarization in mouse pancreatic beta-cells (Rorsman, P., and G. Trube. 1986. J. Physiol. 374:531-550) was assessed using patch-clamp techniques. A Ca2(+)-dependent component could be identified by its rapid inactivation and sensitivity to the Ca2+ channel blocker Cd2+. This current showed the same voltage dependence as the voltage-activated (Cd2(+)-sensitive) Ca2+ current and contributed 10-20% to the total beta-cell delayed outward current. The single-channel events underlying the Ca2(+)-activated component were investigated in cell-attached patches. Increase of [Ca2+]i invariably induced a dramatic increase in the open state probability of a Ca2(+)-activated K+ channel. This channel had a single-channel conductance of 70 pS [( K+]o = 5.6 mM). The Ca2(+)-independent outward current (constituting greater than 80% of the total) reflected the activation of an 8 pS [( K+]o = 5.6 mM; [K+]i = 155 mM) K+ channel. This channel was the only type observed to be associated with action potentials in cell-attached patches. It is suggested that in mouse beta-cells spike repolarization results mainly from the opening of the 8-pS delayed rectifying K+ channel.

Action Potentials↗

PubMed1990

Endothelin has no positive inotropic effect in guinea-pig atria or papillary muscle.

Endothelin is a recently discovered, highly potent vasoconstrictor peptide. In isolated atria from rat and guinea-pig, endothelin has been reported to elicit a positive inotropic effect. The purpose of the present study was to compare the effects of endothelin on electromechanical coupling in guinea-pig atrial and ventricular muscle. In isolated, electrically driven specimens of atria and papillary muscle, action potentials and isometric contractions were recorded in the basal state and 30 min following non-cumulative exposure to endothelin (100 nM). In the atria, endothelin reduced action potential overshoot and relaxation velocity, and increased resting tension. In the papillary muscle the peptide slightly shortened the duration of the action potential. Endothelin did not affect peak tension, either in the atria or in the papillary muscle. These data contrast with earlier reports on a positive inotropic effect of endothelin in guinea-pig atria.

Action Potentials↗

PubMed1990

Effects of external tetraethylammonium ions and quinine on delayed rectifying K+ channels in mouse pancreatic beta-cells.

1. The whole-cell and outside-out patch configurations of the patch-clamp technique were used to study the mechanisms of block produced by external tetraethylammonium ions (TEA+) and quinine on delayed rectifying K+ channels in mouse pancreatic beta-cells. 2. In whole-cell recordings, TEA+ blocks the delayed outward current (which reflects the activity of delayed rectifying K+ channels by greater than 85%) in a concentration-dependent manner. The block appeared to be 1:1 with a Kd of approximately 1.4 mM at a membrane potential of 0 mV. The value of Kd varied with the membrane potential and there was an e-fold increase for a 70 mV depolarization. 3. Single-channel recordings revealed that delayed rectifying K+ channels have a unitary conductance of 8.5 pS ([K+]1 = 155 mM; [K+]o = 5.6 mM) and a single-channel K+ permeability of 2.8 X 10(-14) cm3 s-1. 4. First latency histograms of channel openings during voltage pulses from -70 to 0 mV peaked after 4 ms. A reaction scheme involving two closed states adequately but not perfectly described the distribution of the first latencies. The openings of the channels were grouped in bursts and the distribution of the closed times required two exponentials with time constants of 2.0 and 13 ms, respectively. The distribution of the open times could be described by a single exponential with a time constant of 25 ms. 5. Channel block produced by TEA+ (1 mM) was associated with a 40% decrease of the single-channel current amplitudes and a reduction in single-channel K+ permeability to 1.9 X 10(-14) cm3 s-1 but did not measurably affect the single-channel kinetics suggesting that the blocking reaction is very rapid. 6. Quinine blocked the whole-cell delayed outward current in a concentration-dependent manner. Half-maximal inhibition was attained at approximately 4 microM and the binding appeared to be 2:1. 7. Single-channel recordings indicated that the inhibition produced by quinine (10 microM) resulted from a decrease in the duration of the openings to a mean value of 6.7 ms. The time constants for the distribution of the closures were increased by approximately 30%. Quinine did not affect the amplitude of the openings. The rate constant of the blocking reaction (kB) was 15 mM-1 ms-1 at 0 mV.

Animals↗

PubMed1990

Block of ATP-regulated and Ca2(+)-activated K+ channels in mouse pancreatic beta-cells by external tetraethylammonium and quinine.

1. The whole-cell and outside-out patch configurations of the patch-clamp technique were used to investigate the effects of extracellular tetraethylammonium ions (TEA+) and quinine on both Ca2(+)-activated and ATP-regulated K+ channels in mouse pancreatic beta-cells. 2. The Ca2(+)-activated K+ channel has a single-channel K+ permeability of 4.7 x 10(-13) cm3 s-1 when recorded with physiological ionic gradients. This value decreased to 2.9 x 10(-13) cm3 s-1 after addition of 0.3 mM-TEA+. 3. Two exponentials with time constants of 0.2 and 4.7 ms were required to describe the distribution of the channel openings suggesting that the Ca2(+)-activated K+ channel has at least two open states. The fast and slow components comprised 16 and 84% of the total number of openings respectively. 4. TEA+ caused a concentration-dependent decrease in the single-channel amplitude and open probability of the Ca2(+)-activated K+ channel. A Kd for the reduction in the mean current of 0.14 mM was observed. The stoichiometry was approximately 1:1. 5. Quinine blocked the Ca2(+)-activated K+ channel in a concentration-dependent manner. Half-maximal block was observed at 0.10 mM and binding was 1:1. Inhibition by 20 microM-quinine was not associated with a decrease in channel amplitude but markedly reduced the lifetime of the channel openings. Two exponentials, with time constants of 0.5 and 1.3 ms, were required to describe the channel openings. The rapid component contained 55% of the events. 6. TEA+ reduced the single-channel amplitude of the ATP-regulated K+ channel in a concentration-dependent manner. Kd for the block was 22 mM and the binding approximately 1:1. The block was not associated with changes in the open probability or channel kinetics. Two exponentials were required to describe the distribution of the open times. The time constants for the fast and slow components were approximately 2 and approximately 20 ms respectively. The rapid component accounted for approximately 35% of the events. 7. Quinine (10-20 microM) almost abolished activity of the ATP-regulated K+ channels. Inhibition was characterized by slow onset and reversibility but not associated with a change in the appearance of the single-channel events. Quinine-induced block could not be reversed by diazoxide. 8. We conclude that TEA+ produces rapid block of both Ca2(+)-activated and ATP-regulated K+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine Triphosphate↗

PubMed1989

Glucose-inhibition of glucagon secretion involves activation of GABAA-receptor chloride channels.

The endocrine part of the pancreas plays a central role in blood-glucose regulation. It is well established that an elevation of glucose concentration reduces secretion of the hyperglycaemia-associated hormone glucagon from pancreatic alpha 2 cells. The mechanisms involved, however, remain unknown. Electrophysiological studies have demonstrated that alpha 2 cells generate Ca2+-dependent action potentials. The frequency of these action potentials, which increases under conditions that stimulate glucagon release, is not affected by glucose or insulin. The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is present in the endocrine part of the pancreas at concentrations comparable to those encountered in the central nervous system, and co-localizes with insulin in pancreatic beta cells. We now describe a mechanism whereby GABA, co-secreted with insulin from beta cells, may mediate part of the inhibitory action of glucose on glucagon secretion by activating GABAA-receptor Cl- channels in alpha 2 cells. These observations provide a model for feedback regulation of glucagon release, which may be of significance for the understanding of the hypersecretion of glucagon frequently associated with diabetes.

Animals↗

PubMed1989

Demonstration of A-currents in pancreatic islet cells.

Voltage-activated K+ currents resistant to TEA but blockable by 4-AP were recorded from mouse pancreatic islet cells. These currents first become observable during depolarizations to voltages more positive than -40 mV, reaching a peak amplitude of 120 +/- 34 pA at +6 mV (n = 4), display rapid turn on (tau = 3.3 +/- 1.1 ms at +6 mV) and inactivate completely within 250 ms (tau = 65 +/- 5 at +6 mV). The current is subject to steady-state inactivation. The midpoint (Vh) of the inactivation curve (h infinity) was observed at -72 +/- 2 mV. The properties of this current resemble those reported for the A-current in neurons.

Animals↗

PubMed1989

Failure of glucose to elicit a normal secretory response in fetal pancreatic beta cells results from glucose insensitivity of the ATP-regulated K+ channels.

Fetal pancreatic beta cells demonstrate a deficient insulin release in response to glucose, but the underlying mechanism at the cellular level is unknown. By using beta cells from 21-day fetal rats we made an attempt to clarify the mechanism(s) behind this reduced glucose response. In addition to measuring insulin release, glucose metabolism, and cellular ATP content, ATP-regulated K+ channels (G channels) and voltage-activated Ca2+ currents were investigated with the patch-clamp technique. It was thus demonstrated that the ATP-regulated K+ channels in fetal beta cells were not sensitive to glucose but otherwise had similar characteristics as those of adult beta cells. Also, the characteristics of the voltage-activated Ca2+ currents were similar in adult and fetal beta cells. However, as judged from measurements of both glucose oxidation and glucose utilization, glucose metabolism was impaired in fetal beta cells. In addition, there was no increase in the ATP content, even when the cells were stimulated for 30 min. It is therefore concluded that the attenuated glucose-induced insulin release in fetal pancreatic beta cells is due to an immature glucose metabolism resulting in impaired regulation of the ATP-sensitive K+ channels. These findings may be relevant to the understanding of the deficient stimulus-secretion coupling associated with non-insulin-dependent diabetes.

Adenosine Triphosphate↗

PubMedDate not supplied

Does galanin inhibit insulin secretion by opening of the ATP-regulated K+ channel in the beta-cell?

The intrapancreatic neuropeptide galanin potently inhibits glucose-induced insulin secretion. This effect is in part due to a repolarization of the beta-cells and ensuring reduction in the cytoplasmic free Ca2+ concentration, [Ca2+]i. We propos that galanin inhibition of beta-cell action potentials is associated with the appearance of ATP-regulated K+ channels. Galanin opens K+ channels in a patch membrane when applied to the external solution in the cell-attached patch configuration. However, galanin does not detectably increase K+ permeability during whole-cell experiments, even when GTP was included in the internal solution. Our findings are not consistent with a direct effect of galanin on the K+ channels, but rather indicate that the effect of the neuropeptide is mediated by some intracellular coupling factor(s).

Adenosine Triphosphate↗

PubMedDate not supplied

Ion channels, electrical activity and insulin secretion.

The insulin-secreting pancreatic beta cell is electrically excitable and changes in the membrane potential play an important role in coupling the metabolism of glucose (and other nutrient secretagogues) to the discharge of the insulin-containing granule. The application of the patch-clamp technique, which permits the recordings of the minute currents associated with the opening of individual ion channels, to pancreatic islet cells has revolutionized our understanding of the beta cell electrophysiology. Here we review some of the recent progress in the field. The properties of functionally important ion channels are described and their possible roles are discussed.

Adenosine Triphosphate↗