Effects of sympathomimetic amines on 45Ca efflux from liver slices

Cyclic AMP stimulates Ca2+ entry in rat hepatocytes by interacting with the plasma membrane carriers involved in receptor-mediated Ca2+ influx

The regulation of Ca2+ influx in rat hepatocytes by glucagon and cyclic AMP (cAMP) was investigated. Exposing hepatocytes to glucagon resulted in an increase in the initial rate of Ca2+ entry. The concentrations of glucagon producing half-maximal and maximal stimulation of Ca2+ entry were 10(-10) and 10(-8) M, respectively. A similar stimulation of Ca2+ influx was obtained in cells exposed to cAMP analogues or to forskolin. Exposing hepatocytes suspended in nominally Ca(2+)-free medium to glucagon for 3 min produced a 9% decrease in the size of the vasopressin-sensitive Ca2+ pool; in contrast, N6,2'-O-dibutyryladenosine 3':5'-cyclic monophosphate (Bt2cAMP) slightly augmented the size of this pool. Glucagon and Bt2cAMP synergized the initial vasopressin-stimulated Ca2+ and Mn2+ influx rates, but only moderately increased the initial rate of Ca2+ entry after thapsigargin addition. The glucagon- and Bt2cAMP-stimulated Ca2+ influx was inhibited by the same antagonists of the plasma membrane Ca2+ carriers that mediate Ca2+ entry during stimulation by vasopressin. Thus, cAMP does not stimulate Ca2+ entry through either a capacitative type of mechanism or inositol phosphate turnover. The authors' findings instead suggest that cAMP acts directly, or through protein kinase A on the same Ca2+ carriers that are activated by phospholipase C-linked receptor agonists.

Effects of catecholamines on plasma potassium: the role of alpha- and beta-adrenoceptors

The sympathetic nervous system plays an important role in the control of plasma potassium levels. Administration of adrenaline or noradrenaline evokes, in the majority of mammal species a dual response: first a short transient hyperkalaemia, followed by a maintained hypokalaemia. Alpha 1- and alpha 2-adrenoceptors mediate the initial hyperkalaemia through the activation of hepatic Ca(2+)-dependent-K(+)-channels. Stimulation of beta 1- and beta 2-adrenoceptors induces the late hypokalaemia by stimulation of skeletal muscle Na(+)-K(+)-ATPase. Beta 3-adrenoceptor stimulation may also have an effect on plasma potassium control since administration of selective beta 3-adrenoceptor agonists induces a decrease in plasma potassium. The simultaneous infusion of phenyleprine (alpha-adrenoceptor agonist) and isoprenaline (beta-adrenoceptor agonist) increases plasma potassium levels: this effect is several times larger than the algebric summation of the changes in plasma potassium when each agent is infused separately, thus suggesting potentiation. The physiological (changes in cell volume and function secondary to changes in ion fluxes) and clinical implications (pathophysiological conditions with hypo or hyperkalaemia, hyperkalaemic periodic paralysis, ventricular arrythmias) of these findings are discussed.

Functional involvement of α1and α2-adrenoceptors in86Rb efflux from liver slices and lipolysis in guinea-pig isolated adipocytes

1. The application of an alpha 1-adrenoceptor agonist, amidephrine, to guinea-pig liver slices increases glucose release and 86Rb efflux. Since prazosin was more potent than yohimbine in inhibiting both responses, alpha 1-adrenoceptors seem to be involved in the effects evoked by the agonist. 2. Clonidine (an alpha 2-adrenoceptor agonist) at doses unable to activate liver glycogenolysis increased 86Rb release and potentiated isoprenaline in promoting 86Rb efflux. Since yohimbine antagonized clonidine in promoting 86Rb efflux, alpha 2-adrenoceptors also seem to control plasmalemmal permeability to 86Rb. 3. The liver slice responses resulting from alpha 1- and alpha 2-adrenoceptor stimulation required extracellular calcium. Calcium absence or the administration of D-600 attenuated the effects of amidephrine on glucose release and 86Rb outflow and Ca2+ excess re-established both responses. D-600 and apamin blocked clonidine-induced 86Rb efflux, suggesting that alpha 2-adrenoceptor stimulation activates calcium dependent K+ channels. 4. alpha 2-adrenoceptors do not appear to mediate antilipolytic effects in guinea-pig fat cells.

Kinetics of the conductance evoked by noradrenaline, inositol trisphosphate or Ca2+ in guinea-pig isolated hepatocytes

1. Guinea-pig hepatocytes respond to noradrenaline (NA, 5-10 microM) with a large membrane conductance increase to K+ and Cl-. The response has a long initial delay (range 2-30 s). Following the delay, the K+ conductance (studied in Cl(-)-free solutions) rises quickly to a peak in 1-2 s and is maintained in the continued presence of NA, though often with superimposed oscillations of conductance. The roles of intracellular Ca2+ and D-myo-inositol 1,4,5-trisphosphate (InsP3) in this complex response have been investigated by rapid photolytic release of intracellular Ca2+ (from Nitr5-Ca2+ buffers) or InsP3 from 'caged' InsP3. 2. A rapid increase of intracellular [Ca2+] produced an immediate membrane conductance increase which rose approximately exponentially to a new steady level, consistent with a direct activation of Ca2(+)-dependent ion channels. 3. Following a pulse of InsP3, conductance rose after a brief delay (range 70-1500 ms) which was shortest at high [InsP3] or if the initial cytosolic [Ca2+] had been raised above normal levels. The maximum conductance produced by InsP3 was similar in each cell to the peak recorded with NA and could be evoked by InsP3 concentrations of 0.5-1 microM. 4. The rates of rise of conductance increased with InsP3 concentration in the range 0.25-12.5 microM (range 10-90%, rise times 90-1000 ms), indicating that InsP3-evoked Ca2(+)-efflux from stores increases with InsP3 concentration in this range. 5. Photochemically released InsP3 and Ca2+ activate at physiological concentrations the same membrane conductances as NA. The results indicate that the long initial delay in NA action occurs prior to or during generation of InsP3. The mechanism of the delay and the subsequent apparently all-or-none conductance increase during NA action are discussed in terms of the high co-operativity in InsP3 and Ca2+ actions and an additional positive feedback step. 6. Evidence was found of a negative interaction between [Ca2+] and InsP3-evoked Ca2+ release. The time course of the recovery of InsP3-evoked Ca2+ release following a rise of cytosolic [Ca2+] suggests that this interaction may be important in regulating oscillatory responses of [Ca2+] during hormonal stimulation of guinea-pig hepatocytes.

Properties of membrane ion conductances evoked by hormonal stimulation of guinea-pig and rabbit isolated hepatocytes

Membrane conductance changes evoked in isolated guinea-pig or rabbit hepatocytes by hormonal stimulation were studied with the whole-cell patch clamp technique. In Cl-containing solutions, noradrenaline (NA), ATP or angiotensin II (AII) evoked an increase of conductance to both K (GK) and Cl (GCl) ions. Activation of GK occurred after a delay of several seconds and was sustained in the presence of hormone. Activation of GCl was transient, lasting several seconds, and arose either at the same time or shortly after the increase in GK. Conductances showed an initial rapid rise and slow oscillatory changes during maintained hormone application. The NA-induced current reversed at -19 mV in Cl solutions, between the equilibrium potentials for chloride (ECl = 0 mV) and potassium ions (EK = -85 mV), and at -75 mV, near EK, in Cl-free solution. In both conditions whole-cell current-voltage curves were linear in the range -100 mV to +40 mV. The conductance increase produced by NA to Cl- ions was about 50 nS, that to K+ ions was 6 nS. The potassium conductance increase was abolished by the polypeptide toxin apamin (50 nM). An increase in membrane current noise was associated with NA-evoked outward K+ current and blocked by apamin. Spectral analysis gave estimates of the elementary K channel conductance of 1.7 pS. Power spectra were fitted by two Lorentzian components, with average half-power frequencies of 2 and 190 Hz. These results are discussed in relation to the single-channel properties and indicate that the open probability of K channels during the NA response is high. In Cl solutions, with apamin to block the K conductance, no increase in current noise was detected during the large Cl conductance evoked by NA. This suggests either that Cl channels are of very low unitary conductance (less than 1 pS) or that Cl transport is due to a membrane carrier. The complex time-course of hormonally evoked conductances is not due to the properties of ion conductances per se but probably to underlying changes of intracellular second-messenger concentration.

Role of alpha 1- and alpha 2-adrenoceptors in catecholamine-induced hyperglycaemia, lipolysis and insulin secretion in conscious fasted rabbits

1. In conscious fasted rabbits an intravenous infusion of clonidine (2 micrograms kg-1 min-1) induced hyperglycaemia. The increase in blood glucose was accompanied by an inhibition of insulin secretion and basal lipolysis. 2. Yohimbine infused at a rate of 20 micrograms kg-1 min-1 suppressed clonidine-induced hyperglycaemia and blocked the inhibitory effect on insulin secretion mediated by the alpha 2-adrenoceptor agonist. 3. The intravenous infusion of amidephrine (10 micrograms kg-1 min-1) induced an increase in insulin secretion in the absence of patent hyperglycaemia. Prazosin, 0.3 mg kg-1 s.c. selectively antagonized the effect of amidephrine on insulin secretion. 4. Isoprenaline infusion (4.4 micrograms kg-1 min-1) evoked a significant increase in blood glycerol and immunoreactive insulin plasma levels. Both responses were clearly attenuated when alpha 2-adrenoceptors were simultaneously stimulated by selective (clonidine) and less selective (phenylephrine, 20 micrograms kg-1 min-1) agonists. 5. Amidephrine infusion did not induce appreciable changes in blood glycerol nor did it modify, isoprenaline-induced lipolytic response. 6. Simultaneous infusion of isoprenaline and amidephrine induced a remarkable increase in insulin secretion. 7. It is concluded that in normal fasted rabbits stimulation of alpha 2-adrenoceptors depresses basal and beta-adrenoceptor mediated lipolysis and insulin secretion. On the other hand, selective stimulation of alpha 1-adrenoceptors does not affect lipolysis but induces insulin release. Simultaneous stimulation of alpha 1- and beta-adrenoceptors potentiates the insulin secretory response.

The effect of noradrenaline on the ion permeability of isolated mammalian hepatocytes, studied by intracellular recording

1. The influence of noradrenaline on the membrane potential and conductance of isolated guinea-pig and rabbit hepatocytes in short-term (2-8 h) tissue culture has been studied by intracellular recording. 2. Resting hepatocytes had linear current-voltage relationships, with input resistances of 166 and 216 M omega in guinea-pig and rabbit cells respectively. The recorded membrane potential was -18 mV in each species, though the true resting potential is likely to have been up to 10 mV greater. 3. The hepatocytes sometimes slowly hyperpolarized during intracellular recording, and this was associated with a fall in membrane resistance, and an increase followed by a decrease in membrane potential noise. These effects were abolished by quinine (200 microM) but not by apamin (50 nM), and are attributable to a K+ conductance activated by cell swelling. 4. Noradrenaline (2 microM, in the presence of propranolol at 1 microM) was applied to individual hepatocytes by pressure ejection (puffer pipette technique). After a short latency, the cells hyperpolarized by a mean of 18 mV in both guinea-pig and rabbit preparations. This was associated with a large rise in membrane conductance (50 nS in guinea-pig, 54 nS in rabbit cells). The reversal potential for this action was -38 mV. 5. The experiments were repeated in the presence of apamin (50 nM) to block the Ca2+-dependent K+ permeability which noradrenaline activates in these cells. Noradrenaline still caused some hyperpolarization and a substantial increase (approximately 40 nS) in conductance, with a reversal potential (Er) of -31 mV. This can be attributed to an increase in Cl- conductance. 6. In keeping with this interpretation, noradrenaline applied in the absence of Cl- (replaced by isethionate or gluconate) caused a much greater hyperpolarization (58 mV in guinea-pig, 40 mV in rabbit cells) associated with a smaller rise in conductance (approximately 12 nS). Er for this action was -95 mV (guinea-pig) and -68 mV rabbit), suggesting that the conductance increase was now mainly to K+. 7. The magnitudes of the conductance changes produced by noradrenaline under the various experimental conditions suggest that the increase in the conductance to Cl- (delta GCl) is 3-fold greater than that to K+ (delta GK). 8. The activation of delta GCl occurs either at the same time as delta GK, or (in ca. one cell in ten) a few seconds later.(ABSTRACT TRUNCATED AT 400 WORDS)

Alpha-adrenoceptor involvement in catecholamine-induced hyperglycaemia in conscious fasted rabbits

In conscious fasted rabbits an intravenous infusion of phenylephrine (20 micrograms kg-1 min-1) induced hyperglycaemia. The increase in blood glucose was accompanied by a modest increase in insulin secretion and a reduction of liver glycogen. Muscle glycogen and blood lactate levels were not altered by treatment with phenylephrine. Prazosin, 1 mg kg-1 s.c., partially attenuated phenylephrine-induced hyperglycaemia. Phenoxybenzamine infusion (16.6 micrograms kg-1 min-1) for 15 min suppressed the increase in blood glucose and the reduction in liver glycogen evoked by phenylephrine. This alpha-adrenoceptor blocker also clearly attenuated the blood glucose elevation observed on infusing adrenaline at 0.3 microgram kg-1 min-1. Blockade by phenoxybenzamine of phenylephrine- and adrenaline-induced hyperglycaemia was not accompanied by a significant increase in immunoreactive insulin plasma levels. Yohimbine infused at a rate of 20 micrograms kg-1 min-1, also completely blocked phenylephrine-induced hyperglycaemia. This suppressor effect was accompanied by a marked rebound in insulin secretion. It is concluded that in normal fasted rabbits stimulation of alpha-adrenoceptors induces hyperglycaemia. The increase in blood glucose depends mainly on liver glycogenolysis and inhibition of insulin secretion. Separate blockade of each component suffices to reduce alpha-adrenoceptor-mediated hyperglycaemia.

Interactions between receptors that increase cytosolic calcium and cyclic AMP in guinea-pig liver cells

The action of agonists which increase the K+ permeability of liver cells was studied by using a K+-sensitive electrode to record the net movement of K+ between guinea-pig isolated hepatocytes and their suspension medium. Two types of agonist were examined. Type 1 comprised angiotensin II, ATP, noradrenaline and amidephrine, all of which are thought to raise cytosolic Ca2+ in hepatocytes. The Type 2 agonists were isoprenaline and glucagon, which activate adenylate cyclase. Each type of agonist initiated K+ loss from the hepatocytes though the response to Type 2 agonists was more variable than that to Type 1, and sometimes absent. Simultaneous application of a small concentration of an agonist from each class caused a loss of K+ which was much larger than the sum of that seen with each agonist alone, i.e. potentiation occurred. The alpha-adrenoceptor antagonist, WB 4101, abolished potentiation if applied after an alpha-agonist, and before a Type 2 agonist, showing that both receptors have to be active for potentiation to occur. Simultaneous application of a maximal concentration of each type of agonist caused a larger loss of K+ (approximately 17% of the cell total within 45 s) than did a maximal concentration of a Type 1 agonist alone (approximately 10%). Since the K+ loss caused by these agonists is thought to be a consequence of a rise in cytosolic Ca2+, the influence of both types of agonist on 45Ca and 42K efflux from guinea-pig liver slices was studied. The effect of isoprenaline on 45Ca and 42K efflux became much greater following a previous application of the alpha-adrenoceptor agonist, amidephrine. In the presence of apamin, the potentiated effect of isoprenaline on 42K efflux was greatly reduced whereas that on 45Ca efflux was little affected. The effects of Type 1 and Type 2 agonists separately and together on the cyclic AMP content of isolated hepatocytes were examined. Type 2 agonists increased cyclic AMP in the expected way. The increase became slightly smaller, if anything, when a Type 1 agonist was applied at the same time. Hence potentiation could not be ascribed to changes in cyclic AMP formation. Possible mechanisms for potentiation are discussed. Our evidence suggests, albeit indirectly, that it is a consequence of an interaction between the effects of the two types of agonist on cytosolic Ca2+.

Alpha-adrenergic stimulation of potassium efflux in guinea-pig hepatocytes may involve calcium influx and calcium release

Either 86Rb or 42K appears to be a useful marker for monitoring the movement of cellular K in dispersed guinea-pig hepatocytes. Alpha-adrenergic stimulation of perifused hepatocytes causes a biphasic increase in 86Rb or 42K efflux from hepatocytes previously equilibrated with radio-isotope. The first phase is a large transient (about 5 min) increase which is followed by a slowly falling phase of release. Alpha-adrenergic stimulation of hepatocytes perifused with medium containing no added Ca plus 0.1 mM-EGTA evokes only the transient increase in 86Rb efflux. The addition of Ca to the medium in the continued presence of agonist restores the second phase of the response. Both phases of the response appear to be mediated by alpha 1-receptors. The magnitude of the second phase is dependent upon the concentration of Ca added to the perifusion medium. Other agonists that are believed to act by mobilizing Ca give similar results in this system. Angiotensin II, ATP and A23187 stimulate a transient increase in 86Rb efflux without extracellular Ca present, with the second phase of the response appearing upon the addition of Ca to the medium. These results suggest that the initial transient phase of 86Rb efflux, which is independent of extracellular Ca, is stimulated by Ca released from an intracellular pool. The second phase, which occurs only in the presence of extracellular Ca, is probably a result of Ca influx into the cell.

Calcium-activated potassium channels in liver cells

Activation of certain membrane receptors increases the concentration of Ca2+ in the cytosol of hepatocytes. Since in most species these cells possess a PK(Ca) mechanism, the outcome is a rise in PK. This can be blocked by quinine, apamin and certain neuromuscular blocking agents. The binding of labelled apamin to hepatocytes has been studied under physiological conditions, and the relationship between the binding sites and K+ channels is discussed. The physiological role of the PK(Ca) mechanism in hepatocytes is unclear, though it is largely responsible for 'adrenaline hyperkalaemia'.

Effects of apamin on alpha-adrenoceptor-mediated changes in plasma potassium in guinea-pigs

An intravenous K+-sensitive electrode has been used to monitor plasma [K+] changes induced by adrenaline (1.4-6.8 micrograms kg-1) and amidephrine (14-340 micrograms kg-1) in anaesthetized guinea-pigs. A biphasic response consisting of an initial increase in [K+] followed, within 1 min, by a fall below baseline was observed with both agonists. Apamin (0.4-40 micrograms kg-1) reduced the hyperkalaemic phase of the response to amidephrine in a dose-related, non-competitive manner. The response to adrenaline was also reduced but to a lesser extent. Apamin caused little or no reduction of the hypokalaemic phase of the response to either agonist.

Endothelium-dependent relaxation of the pig aorta: relationship to stimulation of 86Rb efflux from isolated endothelial cells

Bradykinin, adenosine triphosphate (ATP) and acetylcholine each relaxed histamine-contracted strips of pig aorta in a dose-dependent manner. These relaxations were abolished when the endothelium was removed. Relaxation induced by ATP was mimicked by adenosine diphosphate (ADP) but adenosine monophosphate (AMP) and adenosine were about 120 times less potent. Relaxation induced by acetylcholine was antagonized by atropine in a competitive manner, and carbachol induced the same degree of relaxation as acetylcholine, but was about 10 times less potent. The calcium ionophore, A23187, also induced a dose-dependent relaxation of pig aortic strips provided the endothelium was present, suggesting that a rise in the level of ionized calcium within the endothelial cells is one means by which vascular smooth muscle relaxation can be triggered. Bradykinin, ATP, ADP, AMP, adenosine and A23187 each induced a dose-dependent increase in 86Rb efflux from preloaded pig aortic endothelial cells. The dose-response curves for stimulation of 86Rb efflux and for endothelium-dependent relaxation were similar for each individual compound. ADP was equipotent with ATP, but AMP and adenosine were about 120 times less potent. Neither acetylcholine nor carbachol, in concentrations that induce endothelium-dependent relaxation, had any effect on 86Rb efflux from isolated aortic endothelial cells. Lanthanum, which blocks calcium influx, abolished the increases in 86Rb efflux induced by bradykinin and ATP, and the calcium ionophore A23187 was the most effective stimulant of 86Rb efflux, suggesting that the potassium transport induced by these agents is calcium-activated. It is concluded that endothelial responses to bradykinin and ATP can be assessed by monitoring 86Rb efflux, which probably reflects a calcium-activated efflux of potassium associated with the endothelium-dependent vascular relaxation induced by these agents. This pathway is apparently not involved in endothelial responses to acetylcholine.

Mobilization of cellular calcium-45 and lead-210: effect of physiological stimuli

Isolated rat hepatocytes in primary culture were used as a model system to evaluate the effects of selected hormones and culture conditions on the efflux of calcium-45 and lead-210 from cells labeled with these isotopes. Alpha-adrenergic stimuli, angiotensin, vasopressin, dibutyryl adenosine 3',5'-monophosphate, and reduced phosphate concentrations in the medium increased the efflux of calcium-45 and lead-210. Glucagon and insulin had no effect, but increased phosphate concentrations decreased the efflux of both isotopes. Experiments with hepatocytes cultured in a medium free of calcium and lead demonstrated that the increased efflux of calcium-45 and lead-210 induced by hormones was the result of mobilization of the ions from intracellular stores. The data indicate that the physiological stimuli that mobilized calcium ions also mobilized lead ions, and that the mobilized lead would be available to interact with calcium-mediated cell functions.

Calcium and the action of adrenaline, adenosine triphosphate and carbachol on guinea-pig taenia caeci

1. The action of adrenaline (in the presence of propranolol; 3 x 10(-6) M), adenosine triphosphate (ATP) and carbachol on guinea-pig taenia caeci, and the interaction between these agonists, was studied by measuring changes in membrane potential using the sucrose-gap method in quiescent preparations at 22 degrees C.2. A sustained hyperpolarization was caused by addition of adrenaline (3 x 10(-6) M) and by applying adenosine triphosphate (ATP; 4 x 10(-4) M) for 5 min in Krebs solution. In calcium-free medium containing EGTA (0.4 mM) and high magnesium (6.2 mM), both the alpha-agonist and ATP caused a transient hyperpolarization which passed off within 5 min, although the agonist was still present.3. The transient hyperpolarization evoked by these agonists in the absence of calcium could be evoked only once. The response was restored after exposure to high calcium, (40 mM for 2 s, or 10 mM for 30 s). The maximum amplitudes of the hyperpolarization caused by adrenaline or ATP after exposure to high calcium (40 mM or 10 mM) were similar, while the maximum hyperpolarization after application of 2.5 mM-calcium was smaller.4. The area of the maximal response evoked by adrenaline or ATP was independent of the exposure time to calcium-free solution after removal of the extracellular calcium (20 min). The sum of the areas of a first submaximal response, obtained by applying adrenaline for less than 5 min to the calcium-free solution (20 min), and of the second response (5 min application) elicited after continuing in calcium-free medium for another 8 min, was constant.5. In the presence of the bee toxin apamin (10(-7) M), addition of ATP (4 x 10(-4) M) caused depolarization of the membrane both in the presence and absence of external calcium. These responses were not blocked in low sodium solution (22.7 mM) but were reduced by the calcium antagonist D600 (2 x 10(-5) M).6. In calcium-free conditions the alpha-response to adrenaline was decreased by a preceding addition of ATP and vice versa. Abolition of the ATP response (4 x 10(-4) M) by adrenaline (10(-5) M) was prevented by blocking the alpha-receptors with phentolamine (2 x 10(-5) M).7. Carbachol (5 x 10(-7)-5 x 10(-5) M) depolarized the muscle cells in calcium-free medium; a second addition of carbachol also caused depolarization, the amplitude being lower. The carbachol depolarization was dependent on the exposure time to calcium-free solution.8. The adrenaline response was reduced by about 25% by carbachol if applied previously, independent of the carbachol concentration (5 x 10(-7)-5 x 10(-5) M). The carbachol response, however, was not affected if preceded by the alpha-response.9. It is concluded that ATP and the alpha-agonist, after binding to their receptor sites, activate the same mechanism, which is mobilization of calcium from the same membrane compartment to open potassium channels, causing hyperpolarization of the muscle cell membrane; the hyperpolarization is transient or sustained in nature depending on the availability of external calcium to replenish the calcium compartment localized in the membrane. This adrenaline and ATP-sensitive calcium compartment is distinct from that which is sensitive to carbachol.

Isoprenaline- and noradrenaline-induced hyperpolarization of guinea-pig liver cells

1 Effects of pretreatment with isoprenaline (Isop) or noradrenaline (NA) and various ionic environments on the NA-induced or Isop-induced hyperpolarization of guinea-pig liver cells were investigated by means of a microelectrode technique.2 NA (5.9 x 10(-6) M) decreased the membrane resistance, and hyperpolarized the membrane with or without generation of an initial transient small depolarization. The NA-induced initial depolarization was not dependent on the membrane potential and was increased by Isop (4.0 x 10(-6) M) or glucagon (10(-7) M).3 In Ca-free solution, the NA-induced hyperpolarization became transient and a continuous depolarization followed in the presence of NA. Repetitive application of NA resulted in a complete disappearance of the NA-induced hyperpolarization and was replaced by a slowly developing depolarization with or without generation of the initial transient depolarization. In excess [Ca](o), the NA or Isop-induced hyperpolarization was increased.4 Both Isop and glucagon hyperpolarized the membrane and decreased the membrane resistance, to various degrees. Repetitive application of Isop or glucagon resulted in the disappearance of both Isop and glucagon-induced hyperpolarizations. Pretreatment with NA not only resulted in a recovery of both Isop and glucagon-induced hyperpolarizations, but also extensively enhanced the hyperpolarization.5 After pretreatment with Isop, the NA-induced hyperpolarization was decreased in amplitude and duration and was followed by a slowly developing depolarization. After repetitive application of Isop, NA produced only depolarization of the membrane, and in these conditions, Isop, glucagon or ATP also depolarized the membrane. These depolarizations were reversed to hyperpolarizations by pretreatment with excess [Ca](o).6 After treatment with Na-deficient solution, NA depolarized the membrane and decreased the membrane resistance. Excess [Ca](o) restored the NA-induced membrane response from one of depolarization to one of hyperpolarization.7 In the presence of tetraethylammonium 10mM, the NA-induced hyperpolarization became transient or ceased and depolarization occurred with a reduction in the membrane resistance.8 It is postulated that both NA and Isop increase the free [Ca](i) by releasing bound Ca from storage sites and consequently an increase in K conductance follows. NA but not Isop promotes Ca-influx which replenishes the storage site. In Ca-depleted conditions, NA does not elevate the free [Ca](i) to a threshold concentration required for hyperpolarization, probably because NA induces a small release of Ca from storage sites.

Effects of quinine and apamin on the calcium-dependent potassium permeability of mammalian hepatocytes and red cells

1. K-sensitive electrodes placed in the extracellular fluid have been used to show that ATP and noradrenaline cause a rapid loss of up to 10% of the K content of isolated guinea-pig hepatocytes. 2. The hypothesis tha this response is a consequence of a rise in the K permeability of the hepatocyte membrane triggered by an increase in cytosolic Ca is supported by the finding that the divalent cation ionophore A23187 also initiated K loss, in this instance of up to 20-25% of the amount in the cells. 3. Under similar conditions A23187 caused a transient increase, followed by a larger decrease, in the 45Ca content of guinea-pig hepatocytes equilibrated with this isotope. The decrease alone was seen with ATP and noradrenaline. 4. Quinine (1 mM) and the bee venom neurotoxin apamin (10 nM) greatly reduced the effect of ATP, noradrenaline and A23187 on K content without affecting the changes in 45Ca movement. 5. Apamin (10 nM) also abolished the increase in 42K efflux which follows the application of the alpha-adrenoceptor agonist amidephrine to rabbit liver slices; the concurrent rises in 45Ca efflux and glucose release were unaffected. 6. It was concluded that quinine and apamin are able to block either the Ca-dependent K channels present in guinea-pig and rabbit liver cell membranes or the mechanism that controls them. 7. Surprisingly, rat hepatocytes took up rather than lost K when treated with the concentrations of ATP, noradrenaline or A23187 that initiated K loss from guinea-pig cells. This response was greatly reduced by ouabain. 8. Application of large concentrations of A23187 to rat hepatocytes caused K loss associated with cell death. 9. The influence of apamin (10-1000 nM) and quinine (200-1000 micro M) on the Ca-dependent K permeability of red blood cells and ghosts was also studied. Apamin was without effect even when applied to both sides of the ghost membrane, whereas quinine caused inhibition, as reported by others. 10. The results suggest that Ca-dependent K channels or carriers are present in the membranes of liver cells of the guinea-pig and rabbit, but are either lacking or inactive in rat liver. The finding that apamin blocks this mechanism in hepatocytes but not in erythrocytes may mean that the channels differ in these cells.

Calcium and the alpha-action of catecholamines on guinea-pig taenia caeci

1. The involvement of calcium in the alpha-action of adrenaline on guinea-pig taenia caeci was studied by measuring the changes in membrane potential and muscle contraction, using the sucrose-gap method, and by determining the (42)K efflux, in the presence of a beta-blocker (propranolol, 1.8 x 10(-6)m).2. In the presence of extracellular calcium, the hyperpolarization caused by adrenaline (3 x 10(-6)m) was sustained during the period of its application (5 min), both in active preparations (at 36 degrees C) and in quiescent muscle (22 degrees C). In the absence of calcium, adrenaline caused a transient hyperpolarization which was smaller at 36 degrees C than at 22 degrees C and passed off within 5 min, while adrenaline was present.3. Both the sustained and the transient hyperpolarization were associated with an increase in (42)K efflux which had a similar time course. (42)K flux measurements were made in depolarized tissue (52.8 mm-potassium), in which the effect was consistent and more pronounced than in polarized muscle (2.8 mm-potassium).4. The transient hyperpolarization which is resistant to calcium removal and EGTA (0.1-2.0 mm) could be evoked only once but, following a short exposure to calcium (2.5 mm) for 20 sec and readmission of calcium-free medium, it was restored.5. The sustained and the transient hyperpolarization and the increase in (42)K efflux were abolished by the alpha-antagonist phentolamine (10(-5)m); their amplitude was dependent on the adrenaline concentration in the range 10(-7) to 3 x 10(-6)m, and both responses persisted in the absence of sodium or chloride.6. The hyperpolarization and the increase in (42)K efflux were greater at higher external calcium concentrations (0.3-2.5 mm).7. Cobalt (0.6 mm), D600 (2.5 x 10(-5)m) and the bee toxin apamin (10(-7)m) reduced the alpha-response.8. In the presence of apamin, in calcium-containing solution, the sustained hyperpolarization caused by adrenaline was preceded by, or converted to, depolarization, spike discharge and contraction.9. The depolarizing effect of adrenaline in the presence of apamin persisted in sodium-free or chloride-free medium, but was blocked in the absence of calcium and diminished by cobalt and D600.10. It is concluded that the alpha-response of guinea-pig taenia caeci consists of two components, both involving calcium. First, the activation of alpha-receptors increases calcium entry, which leads to the opening of potassium channels, a sustained hyperpolarization and inhibition of muscle activity. Secondly, in the absence of external calcium, a transient hyperpolarization is revealed, presumably due to the release of bound calcium from a limited cellular store which can be replenished by addition of external calcium, and this leads to an increase in potassium permeability.

The effect of glucagon on the kinetics of hepatic mitochondrial calcium uptake

Previous work by this and other laboratories has shown that glucagon administration stimulates calcium uptake by subsequently isolated hepatic mitochondria. This stimulation of hepatic mitochondrial Ca2+ uptake by in vivo administration of glucagon was further characterized in the present report. Maximal stimulation of mitochondrial Ca2+ accumulation was achieved between 6-10 min after the intravenous injection of glucagon into intact rats. Under control conditions, Ca2+ uptake was inhibited by the presence of Mg2+ in the incubation medium. Glucagon treatment, however, appeared to obliterate the observed inhibition by Mg2+ of mitochondrial Ca2+ uptake. Kinetic experiments revealed the usual sigmoidicity associated with initial velocity curves for mitochondrial calcium uptake. Glucagon treatment did not alter this sigmoidal relationship. Glucagon treatment significantly increased the V max for Ca2+ uptake from 292 +/- 22 to 377 +/- 34 nmoles Ca2+/min per mg protein (n = 8) but did not affect the K 0.5, (6.5-8.6 microM). Since the major kinetic change in mitochondrial Ca2+ uptake evoked by glucagon is an increase in V max, the enhancement mechanism is likely to be an increase either in the number of active transport sites available to Ca2+ or in the rate of Ca2+ carrier movement across the mitochondrial membranes.

The hepatic adrenergic receptors

The presence of both alpha- and beta-adrenergic receptors in liver designated the hepatic plasma membrane as a useful tool for the elucidation of the mechanisms by which the hormonal signal is transferred through the membrane via a coupling system to an amplifying entity. This process is well documented for the beta-adrenergic receptor which is linked to adenylate cyclase, whereby it modulates the cyclic AMP level. Much less is known about the alpha-adrenergic receptor. Recently, two factors have contributed to a renewed interest in alpha- and beta-adrenergic receptors in liver: i) The fact that activation of glycogenolysis in isolated liver parenchymal cells by epinephrine may be mediated by either alpha- or beta-adrenergic receptors, depending on the species or on the state of nutrition, and not only by beta-adrenergic receptors as previously thought. ii) The existence of specific adrenergic agonists and antagonists radiolabeled to a high specific activity which has permitted the characterization of adrenergic receptors in terms of nature, number, affinity and regulation. The present review will be devoted to the recent progress made in the physiological, pharmacological and biochemical characterization of alpha- and beta-adrenergic receptors in the liver.

The action of apamin on guinea-pig taenia caeci

Apamin (10(-7) M), a substance extracted from bee venom (apis mellifica) causes stimulation of the taenia caeci as seen from an increase in spike activity. The inhibitory effect of ATP or adrenaline (Adr) was reflected by hyperpolarization of the muscle cell, cessation of spike activity and relaxation of the muscle. The 42K efflux and the membrane conductance were enhanced in the presence of these substances. Apamin converted the hyperpolarization caused by ATP or Adr into a transient depolarization which produced contraction of the muscle cells. The changes in membrane conductance and 42K efflux were diminished by the bee toxin. Furthermore, the potassium-dependent phase of the action potential was lengthened by apamin. Reduction of the extracellular chloride or sodium concentration, blockade of the nervous system by TTX (3 x 10(-7) M) or inhibition of spike activity by D600 (3 x 19(-6) M) did not affect the excitatory and blocking action of apamin. A high concentration of the calcium antagonist D600 (10(-4) M) or omission of extracellular calcium was needed to reduce the transient depolarization evoked by ATP or Adr in the presence of apamin. It is concluded that apamin prevents the opening of the ATP- and Adr-sensitive and voltage-dependent potassium channels in guinea-pig taenia caeci.

Characterization of the hormone-sensitive Ca2+ uptake activity of the hepatic endoplasmic reticulum

The characteristics and kinetics of calcium uptake activity were studied in isolated hepatic microsomes. The sustained accumulation of calcium was ATP- and oxalate-dependent. Glucagon increased microsomal Ca2+ uptake upon either in vivo injection, or in vitro perfusion of the hormone in the liver. In contrast, the effect of insulin depended on the route of administration. Calcium accumulation by subsequently isolated hepatic microsomes increased when insulin was injected intraperitoneally whereas it decreased when the hormone was perfused directly into the liver. These effects of glucagon and insulin were dose dependent. When insulin was added to the perfusate prior to the addition of glucagon, insulin blocked the glucagon-stimulated increase in microsomal Ca2+ uptake. Cyclic AMP mimicked the effect of glucagon on microsomal Ca2+ accumulation when the cyclic nucleotide was perfused into the liver. The effects of glucagon and insulin on the kinetics of hepatic microsomal Ca2+ uptake were investigated. In microsomes isolated from perfused rat livers treated with glucagon the V of the uptake was significantly increased over the control values (12.2 vs. 8.6 nmol Ca2+ per min per mg protein, P less than 0.02). In contrast, the addition of insulin to the perfusate significantly decreased the V of Ca2+ uptake by subsequently isolated microsomes (6.8 vs. 8.3 nmol Ca2+ per min per mg protein, P less than 0.05). However, neither hormone had an effect on the apparent Km for Ca2+ (4.1 +/- 0.5 microM) of the reaction. The effect of these hormones on the activity of Ca2+-stimulated ATPase was also studied. No significant changes in either V or Km for Ca2+ of the enzymatic reaction were detected.

Mechanisms involved in alpha-adrenergic phenomena: role of calcium ions in actions of catecholamines in liver and other tissues

Epinephrine and norepinephrine binding sites with the physiological characteristics of alpha-adrenergic receptors have been identified in the plasma membranes of liver and other cells. Interaction of catecholamines with these receptors causes a mobilization of calcium ions from mitochondria and perhaps other intracellular stores in liver cells. In other cells, there may also be influx of extracellular calcium ions. Evidence is presented in support of the hypothesis that the rise in cytosolic calcium ions resulting from these changes is responsible for many of the alpha-adrenergic actions of catecholamines. Possible mechanisms by which activation of alpha-adrenergic receptors causes changes in calcium and other aspects of cellular metabolism are discussed.

Apamin blocks certain neurotransmitter-induced increases in potassium permeability

Apamin is a neurotoxic polypeptide of known structure isolated from bee venom. Shuba and coworkers have recently shown that it abolishes the hyperpolarising action of externally-applied ATP on visceral smooth muscle (guinea pig stomach and taenia coli) as well as the hyperpolarisation (inhibitory junction potential) that follows stimulation of the non-adrenergic inhibitory nerve supply to these tissues. As it has been proposed that ATP is the neurotransmitter involved in the latter response, Vladimirova and Shuba tentatively concluded that apamin is a specific postsynaptic blocking agent of this non-adrenergic, possibly 'purinergic', inhibition. We have confirmed the important observation that nanomolar concentrations of apamin reduce inhibition by ATP and by non-adrenergic nerve stimulation, but further experiments suggest that, rather than acting as a specific blocker of ATP receptors, apamin inhibits the increase in potassium permeability caused by a number of agents, including ATP.

Norepinephrine, vasopressin, glucagon, and A23187 induce efflux of calcium from an exchangeable pool in isolated rat hepatocytes

Isolated rat hepatocytes do not actively accumulate Ca(2+) during prolonged incubation in vitro; however, these cells do exhibit a limited exchange of intracellular with extracellular Ca(2+). The exchangeable pool represents about 2 nmol of Ca(2+) per mg of protein. In medium containing either a low (20 muM) or high (1 mM) concentration of Ca(2+), the divalent cation ionophore, A23187 (at concentrations of 0.03-0.1 nmol/mg of protein), causes release of (45)Ca(2+) from this exchangeable pool but does not allow net influx of extracellular Ca(2+) detectable by the use of a Ca(2+)-sensitive electrode. Like A23187, the hormones norepinephrine, vasopressin, and glucagon (at concentrations that stimulate gluconeogenesis) each induces a similar net efflux of Ca(2+). Treatment with one hormone decreases the subsequent reponse to the others, whereas treatment with A23187 abolishes the hormonal effects upon both Ca(2+) release and gluconeogenesis. The action of norepinephrine, but not of glucagon, upon Ca(2+) efflux is prevented by the alpha-adrenergic antagonist, phenoxybenzamine. The action of norepinephrine is not prevented by the beta-adrenergic antagonist, propranolol. Together these results indicate that the release of Ca(2+) from a common pool of exchangeable Ca(2+) is important to the action of a variety of hormones on hepatocytes. This Ca(2+) pool in the isolated hepatocyte is characterized as being similar in size and having exchange kinetics that are comparable to those reported for the major intracellular pool of Ca(2+) in the intact liver. The possibility that this pool is intramitochondrial calcium is discussed.

Fluxes and distribution of calcium in rat liver cells: kinetic analysis and identification of pools

1. Fluxes and distribution of Ca were studied in the perfused rat liver. Kinetic analysis of (45)Ca exchange revealed three compartments with time constants of 4, 14 and 223 min, pool sizes of 250, 385 and 670 mumole.kg(-1) wet wt. respectively, and one non-exchangeable compartment of 400 mumole.kg(-1).2. (45)Ca uptake by in situ mitochondria followed, as a function of cell loading time, a mono-exponential function with a time constant of 16 min. This suggests that the second compartment may be identified as the intracellular pool of Ca. The calculated cell transmembrane flux of Ca was 28 mumole.kg(-1) min(-1) or 0.17 p-mole.cm(-2).sec(-1).3. The maximum (45)Ca uptake by in situ mitochondria was 2.3 n-mole.mg(-1) of protein which represents, on the basis of 50 mg of mitochondrial protein per g of fresh liver, 115 mumole.kg(-1) or 30% of the cytoplasmic pool. A pool of 10.8 n-mole.mg(-1) protein (or 540 mumole.kg(-1)) of non-exchangeable Ca (at steady state) was probably in the form of Ca phosphate precipitated in the mitochondrial matrix.4. Extracellular Ca pools were studied using competitor of Ca binding (La) or Ca chelators (EGTA). La displaced specifically a homogeneous pool of Ca (tau = 5.1 min) which represented a fraction (55 mumole.kg(-1)) of the rapidly exchangeable Ca (first compartment) without perturbing other external pools. On the other hand, EGTA displaced completely that compartment, and about 85% of the Ca of the third compartment. These results suggest that the first and the major fraction of the third compartments are extracellular. They account for 63% of total exchangeable Ca.5. A model of distribution and exchange of Ca in hepatocytes is proposed.

On the role of calcium as second messenger in liver for the hormonally induced activation of glycogen phosphorylase

We have studied the mode of action of three hormones (angiotensin, vasopressin and phenylephrine, an alpha-adrenergic agent) which promote liver glycogenolysis in a cyclic AMP-independent way, in comparison with that of glucagon, which is known to act essentially via cyclic AMP. The following observations were made using isolated rat hepatocytes: (a) In the normal Krebs-Henseleit bicarbonate medium, the hormones activated glycogen phosphorylase (EC 2.4.1.1) to about the same degree. In contrast to glucagon, the cyclic AMP-independent hormones did not activate either protein kinase (EC 2.7.1.37) or phosphorylase b kinase (EC 2.7.1.38). (b) The absence of Ca2+ from the incubation medium prevented the activation of glycogen phosphorylase by the cyclic AMP-independent agents and slowed down that induced by glucagon. (c) The ionophore A 23187 produced the same degree of activation of glycogen phosphorylase, provided that Ca2+ was present in the incubation medium. (d) Glucagon, cyclic AMP and three cyclic AMP-dependent hormones caused an enhanced uptake of 45Ca; it was verified that concentrations of angiotensin and of vasopressin known to occur in haemorrhagic conditions were able to produce phosphorylase activation and stimulate 45Ca uptake. (e) Appropriate antagonists (i.e. phentolamine against phenylephrine and an angiotensin analogue against angiotensin) prevented both the enhanced 45Ca uptake and the phosphorylase activation. We interpret our data in favour of a role of calcium (1) as the second messenger in liver for the three cyclic AMP-independent glycogenolytic hormones and (2) as an additional messenger for glucagon which, via cyclic AMP, will make calcium available to the cytoplasm either from extracellular or from intracellular pools. The target enzyme for Ca2+ is most probably phosphorylase b kinase.

Interactions between the effects of alpha- and beta-adrenoceptor agonists and adenine nucleotides on the membrane potential of cells in guinea-pig liver slices

1 The beta-adrenoceptor agonist isoprenaline normally causes only a small and inconsistent increase in the membrane potential of cells in guinea-pig liver slices, in contrast to the large hyperpolarizations seen with alpha-agonists. However, after a selective alpha-adrenoceptor agonist has been applied, the response to isoprenaline becomes greatly enhanced. 2 Simultaneous application of small doses of an alpha- and beta-agonist produce hyperpolarizations larger than the sum of the responses to each agent alone. 3 These interactions occur with a range of sympathomimetic amines, including some which are not substrates for various processes for the uptake and inactivation of catecholamines. 4 Hyperpolarizations caused by externally applied cyclic adenosine-3',5'-monophosphate (cyclic AMP) also become larger after application of an alpha-agonist. 5 The adenine nucleotides adenosine 5'-diphosphate (ADP) and adenosine 5'-triphosphate (ATP) hyperpolarize guinea-pig liver cells in the dose range 0.1-1.0 mM. This response is not increased after an alpha-agonist. However, ADP and ATP are themselves able to enhance the response to beta-agonists. 6 These interactions between alpha-agonists, beta-agonists and adenine nucleotides seem to involve steps subsequent to receptor activation. Changes in the intracellular actions of cyclic AMP may be concerned.