Effects of catecholamines, ATP and ionophore A23187 on potassium and calcium movements in isolated hepatocytes

Purinergic signalling in the liver in health and disease

Purinergic signalling is involved in both the physiology and pathophysiology of the liver. Hepatocytes, Kupffer cells, vascular endothelial cells and smooth muscle cells, stellate cells and cholangiocytes all express purinoceptor subtypes activated by adenosine, adenosine 5'-triphosphate, adenosine diphosphate, uridine 5'-triphosphate or UDP. Purinoceptors mediate bile secretion, glycogen and lipid metabolism and indirectly release of insulin. Mechanical stress results in release of ATP from hepatocytes and Kupffer cells and ATP is also released as a cotransmitter with noradrenaline from sympathetic nerves supplying the liver. Ecto-nucleotidases play important roles in the signalling process. Changes in purinergic signalling occur in vascular injury, inflammation, insulin resistance, hepatic fibrosis, cirrhosis, diabetes, hepatitis, liver regeneration following injury or transplantation and cancer. Purinergic therapeutic strategies for the treatment of these pathologies are being explored.

Induction of heme oxygenase-1 and heat shock protein 70 in rat hepatocytes: the role of calcium signaling

Stress response genes including heat shock proteins are induced under a variety of conditions to confer cellular protection. This study investigated the role of calcium signaling in the induction of two stress response genes, heme oxygenase-1/hsp32 and hsp70, in isolated rat hepatocytes. Both genes were induced by cellular glutathione depletion. This induction could be inhibited by BAPTA-AM. Culturing in a calcium-free medium prevented the induction of hsp70 gene expression after glutathione depletion without affecting heme oxygenase-1 gene expression. Thapsigargin increased the gene expression of heme oxygenase-1 but not that of hsp70. Thapsigargin-induced heme oxygenase-1 induction was completely inhibited by BAPTA-AM. Incubation with the Ca²⁺-ionophore A23187 augmented heme oxygenase-1 (two-fold) and hsp70 (5.2-fold) mRNA levels. Our data suggests a significant role of Ca²⁺-dependent pathways in the induction of the two stress genes. An increase in the cytoplasmic Ca²⁺ activity seems to play a key role in the cascade of signaling leading to the induction of the two genes. However, the source of Ca²⁺ that fluxes into the cytoplasm seems to be different. Our data provides evidence for a compartmentalization of calcium fluxes, i.e. the Ca²⁺ flux from intracellular stores (e.g. the endoplasmic reticulum) plays a major role in the induction of heme oxygenase-1. By contrast, Ca²⁺ flux from the extracellular medium seems to be a mechanism initiating the cellular signaling cascade leading to hsp70 gene induction.

Caffeine-induced chloride current in dissociated rat hepatocytes

Current responses to caffeine in single hepatocytes dissociated from adult rat liver were investigated with the conventional whole cell patch-recording configuration. Caffeine produced a sustained inward current (Icaf) with increasing conductance at a holding potential of -40 mV. The reversal potential of Icaf was close to the Cl- equilibrium potential. Icaf was not affected by the internal perfusion of 1,2-bis(2-amino-phenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) or Cs+, whereas the Ca(2+)-activated K+ outward current elicited by A-23187 was inhibited by intracellular BAPTA or Cs+. A 1 mM 3-isobutyl-1-methylxanthine (IBMX) was about equipotent to 1 mM caffeine in inducing the current. Icaf was not modulated by the external application of N-(2-[methylamino]ethyl)-5-isoquinolinesulfonamide (H-8), a cyclic nucleotide-dependent protein kinase inhibitor, or intracellular perfusion with guanosine-5'-O-(2-thiodiphosphate) (GDP beta S) or guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S). It was concluded that caffeine induced an increase in membrane Cl- conductance without utilizing the rise of intracellular free Ca2+ or adenosine 3',5'-cyclic monophosphate (cAMP) and without mediating G protein, suggesting the possible existence of caffeine receptor-Cl- channel complexes on liver plasma membrane.

Effect of tauroursodeoxycholic acid on bile flow and calcium excretion in ischemia-reperfusion injury of rat livers

BACKGROUND/AIMS

Tauroursodeoxycholic acid is known to have a hepatoprotective action in cholestatic disorders. We evaluated whether oral pretreatment with tauroursodeoxycholic acid could protect the liver from ischemia-reperfusion injury, with particular regard to its effect on bile flow and biliary calcium excretion.

METHODS

A 1-hour in vivo ischemia-reperfusion model of 70% of the lobes of rat liver was used. Animals were divided into six groups (each group; n = 8); a non-ischemia sham group (CS), a control group without bile acids (CON), and 4 bile acid groups; 10 mg/kg and 50 mg/kg (U10, U50), taurocholic acid 10 mg/kg (CA10) and tauroursodeoxycholic acid 10 mg/kg (CD10). Bile acids were given orally for 7 days before operation.

RESULTS

Three hours after reperfusion, oral bile acid pretreatment failed to reduce the hepatic ischemia-reperfusion injury biochemically, but histological improvement was observed in the tauroursodeoxycholic acid groups. After reperfusion, tauroursodeoxycholic acid significantly increased bile flow from the ischemic liver, and also significantly increased serum calcium concentration. Although tauroursodeoxycholic acid did not change biliary calcium concentration, it significantly enhanced total biliary calcium output during reperfusion.

CONCLUSION

Thus, tauroursodeoxycholic acid inhibited tissue calcium accumulation and enhanced sinusoidal and biliary calcium output during hepatic ischemia-reperfusion. However, it is still unclear if calcium mobilization is part of the protective mechanisms of tauroursodeoxycholic acid in ischemia-reperfusion injury of the liver.

Activation of the plasma membrane chloride channel by protein kinase C in isolated guinea-pig hepatocytes

1. To assess the nature of the underlying mechanism of noradrenaline-induced increase of Cl- conductances in hepatocytes, macroscopic and unitary currents through noradrenaline-induced Cl- channels were examined in enzymatically isolated guinea-pig hepatocytes using whole-cell, cell-attached and excised inside-out configurations of the patch-clamp technique. 2. When K+ conductances were blocked and the intracellular Ca2+ concentration ([Ca2+]i) was set at 0.1 microM, bath application of noradrenaline activated the time-independent membrane currents under whole-cell voltage-clamp conditions. The current was similarly activated by phorbol ester (PMA), an activator of protein kinase C (PKC), while a specific protein kinase C inhibitor, H-9, reversed PMA activation of the current. The inactive phorbol ester, 4 alpha-phorbol 12-myristate, 13-acetate (alpha PMA), failed to activate the channel. 3. The reversal potential of the PMA-activated current shifted by approximately 60 mV per 10-fold change in the external Cl- concentration, indicating that the current was Cl- selective. Bath application of 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) partially inhibited both the noradrenaline- and PMA-induced currents. 4. In single channel recordings from cell-attached patches, bath application of noradrenaline or PMA induced unitary current activity, the averaged slope conductance of which was 10.1 +/- 1.5 pS (mean +/- S.D.; n = 12) in the noradrenaline-induced current and 9.7 +/- 1.3 pS (n = 7) in the PMA-induced current. The open time distribution was moderately well fitted by a single exponential function with mean open lifetime of 88.5 +/- 10.6 ms (n = 10), while at least two exponentials were required to fit the closed time distributions with a time constant for the fast component of 24.4 +/- 5.8 ms (n = 10) and for the slow component of 316.9 +/- 49.2 ms (n = 10). 5. Bath application of purified PKC to excised inside-out patches activated the channel. The PKC selective inhibitor, PKC(19-36), and DIDS inhibited the PKC-activated channel. 6. These results suggest that PKC can phosphorylate the channel protein or a related structure leading to the activation of Cl- channels in guinea-pig hepatocytes.

Possible mechanism of ciliary stimulation by extracellular ATP: involvement of calcium-dependent potassium channels and exogenous Ca2+

Ciliary motility was examined optically in tissue cultures from frog palate epithelium and frog's esophagus as a function of extracellular concentration of adenosine 5'-triphosphate (ATP) and related compounds. The addition of micromolar concentration of ATP caused a strong enhancement of frequency and wave velocity in the direction of the effective stroke. Since adenosine 5'-[beta,gamma imido]-triphosphate (AMP-PNP), a nonhydrolyzable analog of ATP, produces the same effects, ATP hydrolysis is not required. The overall potency is ATP approximately equal to AMP-PNP greater than ADP much greater than adenosine greater than AMP. It is suggested that both the phosphate and the base moieties are involved in ATP binding. The enhancement of ciliary activity by extracellular ATP is dependent on the presence of extracellular Ca2+, which can be replaced by extracellular Mg2+. The effect of a number of potent inhibitors of the voltage-gated calcium channels on the stimulation of ciliary activity by ATP were examined. No effect was detected in the concentration range within which these agents are specific. On the other hand, quinidine, a potent inhibitor of K+ (calcium-dependent) channels, inhibits the effect of ATP. The following model is suggested: exogenous ATP interacts with a membrane receptor in the presence of Ca2+, a cascade of events occurs which mobilizes intracellular calcium, thereby increasing the cytosolic free Ca2+ concentration which consequently opens the calcium-activated K+ channels, which then leads to a change in membrane potential. The ciliary response to these changes is the enhancement of ciliary activity.

Effect of ionophore A23187 on the membrane permeability in mouse fibroblasts

Addition of the divalent cation ionophore A23187 to transformed mouse fibroblasts (3T6) resulted in an increase in the cell membrane permeability to normally impermeant solutes (e.g., nucleotides). The membrane permeability was assessed by following the efflux of prelabeled adenine nucleotides, the influx of p-nitrophenyl phosphate in cells attached to plastic dishes and reconstitution of intracellular protein synthesis in the presence of exogenously added normally impermeant factors required for macromolecular synthesis. The permeability change of 3T6 cells was found to be dependent on the specific presence of external calcium ion. The permeabilization was found to occur preferably in alkaline pH and specific to certain transformed cells. It is preceded by rapid efflux of K+, influx of Na+ and partial hydrolysis of cellular nucleotides in 3T6 cells. Similar ion fluxes were previously found to precede cell permeabilization by electrogenic ionophores for monovalent ions and by exogenous ATP. Our data suggest that a calcium dependent process caused the K+ release and excess Na+ entry, causing dissipation of the membrane potential and subsequent formation of aqueous channels.

Regulation of transepithelial ion transport and intracellular calcium by extracellular ATP in human normal and cystic fibrosis airway epithelium

1 The role of extracellular nucleotides in regulation of ion transport activities (short circuit current, Isc) of human respiratory epithelia was studied. 2 Application of nucleotides to the apical or basolateral membrane of human nasal epithelium induced a concentration-dependent increase in Isc. 3 The rank order of potency of purine- or pyrimidine-induced changes in Isc of normal human nasal epithelium when applied to the apical membrane (UTP greater than or equal to ATP greater than ATP gamma S greater than 2MeSATP greater than ADP beta S much greater than beta gamma MeATP greater than or equal to alpha beta MeATP) or basolateral membrane (2MeSATP greater than UTP greater than ATP greater than ATP gamma S greater than alpha beta MeATP greater than beta gamma MeATP) is consistent with involvement of a P2 purinoceptor. A similar rank order of potencies was observed for nucleotide effects on intracellular calcium measured by Fura-2 fluorescence using microspectrofluorimetry. 4 Similar nucleotide potency in the regulation of ion transport and intracellular calcium in cystic fibrosis (CF) airway epithelium (UTP greater than or equal to ATP) was observed, suggesting purinoceptors might be used to stimulate ion transport processes that would promote hydration of airway secretions and facilitate their clearance from CF lungs. 5 These data provide evidence for the regulation of ion transport by P2 purinoceptors in normal and cystic fibrosis human airway epithelium.

Stimulation of release of prostaglandin D2 and thromboxane B2 from perfused rat liver by extracellular adenosine

In isolated perfused rat liver, adenosine infusion (50 microM) led to increases in glucose output and portal pressure and a net K+ release of 3.7 +/- 0.21 mumol/g, which was followed by an equivalent net K+ uptake after cessation of the nucleoside infusion. These effects were accompanied by a transient stimulation of hepatic prostaglandin D2 and thromboxane B2 release. The Ca2+ release observed upon adenosine infusion (50 microM) was 23.5 +/- 5.2 nmol/g, i.e. 10-20% of the Ca2+ release observed with extracellular ATP (50 microM). Indomethacin (10 microM) prevented the adenosine-induced stimulation of glucose output and the increase in portal pressure by 79 and 63% respectively, and completely abolished the stimulation of prostaglandin D2 release. The thromboxane A2 receptor antagonist BM 13.177 (20 microM), the phospholipase A2 inhibitor 4-bromophenacyl bromide (20 microM) and the cyclo-oxygenase inhibitor ibuprofen (50 microM) also decreased the glycogenolytic and vasoconstrictive responses of the perfused rat liver upon adenosine infusion by 50-80%. When the indomethacin inhibition of adenosine-induced prostaglandin D2 release was titrated, a close correlation between prostaglandin D2 release and the metabolic and vascular responses to adenosine was observed. These findings suggest an important role for eicosanoids in mediating the nucleoside responses in the perfused rat liver. Since eicosanoids are known to be formed by non-parenchymal cells in rat liver [Decker (1985) Semin. Liver Dis. 5, 175-190], the present study gives further evidence for an important role of eicosanoids as signal molecules between the different liver cell populations.

Vasopressin-stimulated Ca2+ influx in rat hepatocytes is inhibited in high-K+ medium

We examined the effects of K+ substitution for Na+ on the response of hepatocytes to vasopressin, and on the hepatocyte plasma-membrane potential. (1) High K+ (114 mM) had no effect on the initial increase in phosphorylase a activity in response to vasopressin, but abolished the ability of the hormone to maintain increased activity beyond 10 min. With increasing concentrations a decrease in the vasopressin response was first observed at 30-50 mM-K+. (2) High K+ (114 mM) had no effect on basal 45Ca2+ influx, but abolished the ability of vasopressin to stimulate influx. This effect was also first observed at a concentration of 30-50 mM-K+. (3) Increasing K+ had little effect on the plasma-membrane potential until a concentration of 40 mM was reached. With further increases in concentration the plasma membrane was progressively depolarized. (4) Replacement of Na+ with N-methyl-D-glucamine+ depolarized the plasma membrane to a much smaller extent than did replacement with K+, and was also much less effective in inhibiting the vasopressin response. (5) The plasma-membrane potential was restored to near the control value by resuspending cells in normal-K+ medium after exposure to high-K+ medium. The effects of vasopressin on phosphorylase activity were also restored. (6) We conclude that the Ca2+ channels responsible for vasopressin-stimulated Ca2+ influx are closed by depolarization of the plasma membrane.

Regulation of hepatic parenchymal and non-parenchymal cell function by the diadenine nucleotides Ap3A and Ap4A

The diadenine nucleotides diadenosine 5',5"-P1,P3-triphosphate (Ap3A) and diadenosine 5',5"-P1,P4-tetraphosphate (Ap4A) can be released from platelets and were shown to act as long-lived signal molecules. Accordingly, we studied their potential effect on hepatic metabolism. In isolated perfused rat liver, Ap3A and Ap4A increase the portal pressure, lead to a transient net release of Ca2+, complex net K+ movement across the liver plasma membrane and stimulate hepatic glucose output and 14CO2 production from [1-14C]glutamate. These responses resemble that obtained with extracellular ATP. This and studies on the additivity of ATP and Ap4A effects suggest similar mechanisms mediating the ATP and diadenine nucleotide effects in the liver. Ap3A and Ap4A increased the activity of glycogen phosphorylase a in isolated hepatocyte suspensions by about 100%, pointing to a direct effect of these nucleotides on hepatic parenchymal cells. A response of hepatic non-parenchymal cells to diadenine nucleotide infusion is suggested by a marked stimulation of thromboxane and prostaglandin D2 release from perfused liver. Studies with the thromboxane A2 receptor antagonist BM 13.177 (20 microM) show that the pressure and glucose response to the diadenine nucleotides is partially mediated by this thromboxane formation. Studies with retrograde and sequential liver perfusions suggest a less efficient degradation of the diadenine nucleotides during a single liver passage compared to extracellular ATP. The data suggest that Ap3A and Ap4A are potential regulators of hepatic hemodynamics and metabolism, involving complex interactions between hepatic parenchymal cells and hepatic non-parenchymal cells, including eicosanoids as signal molecules.

Mobilization of intracellular calcium by extracellular ATP and by calcium ionophores in the Ehrlich ascites-tumour cell

We have studied the changes of the intracellular free calcium concentration ([Ca2+]i) effected by external ATP, which induces formation of inositol trisphosphate, and by the divalent cation ionophores ionomycin and A23187. Both, ATP (40 microM) and ionophores (1-80 mumol/l cells ionomycin; 20-400 mumol/l cells A23187), produced a transient rise of [Ca2+]i which reached its maximum within 15-30 s and declined near resting values (about 200 nM) within 1-3 min. When the [Ca2+]i peak surpassed 500 nM a transient cell shrinkage due to simultaneous activation of Ca2+-dependent K+ and Cl- channels was also observed. The cell response was similar in medium containing 1 mM Ca2+ and in Ca2+-free medium, suggesting that the Ca mobilized to the cytosol comes preferently from the intracellular stores. Treatment with low doses of ionophore (1 mumol/l cells for ionomycin; 20 mumol/l cells for A23187) depressed the response to a subsequent treatment, either with ionophore or with ATP. Treatment with ATP did also inhibit the subsequent response to ionophore, but in this case the inhibition was dependent on time, the stronger the shorter the interval between both treatments. This result suggests that the permeabilization of Ca stores by ATP is transient and that Ca can be taken up again by the intracellular stores. Refill was most efficient when Ca2+ was present in the incubation medium. Addition of either ATP or ionomycin (1-25 mumol/l cells) to cells incubated in medium containing 1 mM Ca2+ decreased drastically the total cell Ca content during the following 3 min of incubation. In the case of ATP the total cell levels of Ca returned to the initial values after 7-15 min, whereas in the case of the ionophore they remained decreased during the whole incubation period. These results indicate that Ca released from the intracellular stores by either ATP or ionophores is quickly extruded by active mechanisms located at the plasma membrane. They also suggest that, under the conditions studied here, with 1 mM Ca2+ outside, the Ca-mobilizing effect of ionophores is stronger in endomembranes than in the plasma membrane.

Modulation of nuclear rotation in neuronal interphase nuclei by nerve growth factor, by gamma-aminobutyric acid, and by changes in intracellular calcium

Nuclear rotation (NR) is typically measured as motion of nucleoli within nuclei of cells in vitro. This occurs in cycling cells. However, its observation in neurons arrested in interphase indicates that mechanisms related to mitosis are not a prerequisite. We have recently shown that NR occurs in three dimensions within the nuclear space, that it occurs within the space delineated by the outer nuclear membrane and that it includes chromatin domains in addition to nucleoli and have postulated that this motion of chromatin domains is related to changes in gene expression. We now show that exposure of dorsal root, sensory neurons in vitro to nerve growth factor (NGF) or to gamma-aminobutyric acid (GABA), agents which alter gene expression, and to agents causing redistribution of calcium, such as EGTA and the calcium ionophore A23187, significantly alters NR. The NGF increased the mean rate of NR and did so at a time after exposure when activity of RNA polymerases have been shown to rise. Exposure to GABA resulted, within minutes, in shifts of the nucleolus within the three-dimensional space of the nucleus, associated in some neurons with significant, sigmoidal increases in the rate of NR. The calcium ionophore A23187 as well as chelation of extracellular calcium with EGTA similarly increased rates. Importantly, excess calcium, with EGTA remaining present, returned NR of all nucleoli to rates not different from controls. This indicates that the increase in NR seen with EGTA is specific to the chelation of calcium and not an nonspecific response to EGTA. It is difficult to link the action of agents which alter gene expression or transmembrane ion balance with changes in NR. Nevertheless, in support of our hypothesis, the results presented here show that agents known to alter gene expression, alter NR in a temporally coincident manner and that they do so, possibly, by calcium-dependent mechanisms.

Hepatocyte heterogeneity in response to extracellular ATP

1. The metabolic and hemodynamic effects of extracellular ATP in perfused rat liver were compared during physiologically antegrade (portal to hepatic vein) and retrograde (hepatic to portal vein) perfusion. ATP in concentrations up to 100 microM was completely hydrolyzed during a single liver passage regardless of the perfusion direction. 2. The ATP(20 microM)-induced increases of glucose output, perfusion pressure and ammonium ion release seen during antegrade perfusions were diminished by 85-95% when the perfusion was in the retrograde direction, whereas the amount of Ca2+ mobilized from the liver was decreased by only 60%. The maximal rate of initial K+ uptake following ATP was dependent on the amount of Ca2+ mobilized regardless of the direction of perfusion. In the presence of UMP (1 mM), an inhibitor of ATP hydrolysis by membrane-bound nucleotide pyrophosphatase, the effect of the direction of perfusion on the glycogenolytic response to ATP (20 microM) was largely diminished. 3. For a maximal response of glucose output, Ca2+ release and perfusion pressure to extracellular ATP, concentrations of about 20 microM, 50 microM and 100 microM were required during antegrade perfusion, respectively. These maximal responses could also be obtained during retrograde perfusion, but higher ATP concentrations were required (120 microM, 80 microM, above 200 microM, respectively). 4. 14CO2 production from [1-14C]glutamate which occurs predominantly in the perivenous hepatocytes capable of glutamine synthesis was stimulated by extracellular ATP (20 microM); it was only slightly affected by the direction of perfusion. In antegrade perfusions, ATP (20 microM) increased 14CO2 production from 88 to 162 nmol g-1 min-1, compared to an increase from 91 to 148 nmol g-1 min-1 in retrograde perfusion. 5. The data are interpreted to suggest that (a) extracellular ATP is predominantly hydrolyzed by a small hepatocyte population located at the perivenous outflow of the acinus; (b) glycogenolysis to glucose is predominantly localized in the periportal area; (c) contractile elements (sphincters) exist near the inflow of the sinusoidal bed; (d) a considerable portion of the Ca2+ mobilized by ATP is derived from liver cells that do not contribute to hepatic glucose output.

Actions of extracellular UTP and ATP in perfused rat liver. A comparative study

1. In perfused rat liver infusion of UTP and ATP in micromolar concentrations increased the portal pressure, with UTP being three times more effective than ATP at concentrations below 50 microM. Whereas ATP (up to 100 microM) increased oxygen consumption, there was a dose-dependent inhibition of oxygen uptake by UTP. 2. Both nucleotides stimulated hepatic glucose output; however, the time-courses were different. Withdrawal of UTP, but not of ATP (up to 100 microM) caused a further transient, but substantial stimulation of glucose output. 3. ATP led to a transient net K+ uptake by the liver being followed by a K+-release phase. Similar changes were observed with UTP; however, the initial K+ uptake was prolonged compared to ATP (1.9 min versus 3.5 min) and withdrawal of UTP, but not of ATP, stimulated hepatic K+ release markedly. 4. Metabolic and hemodynamic effects comparable to those induced by ATP were obtained with beta- and gamma-thio substituted ATP, whereas beta,gamma-methylene-substituted ATP was much less effective. The characteristic effects of UTP on glucose output, portal pressure and K+ fluxes were preserved during constant infusion of ATP or its beta,gamma-methylene derivative, pointing to additive effects. 5. ATP (20 microM) led to a net Ca2+ release (50-60 nmol/g liver) within 2-3 min. When the extracellular Ca2+ concentration was lowered from 1.25 mM to 0.3 mM, this Ca2+ release was increased to about 110 nmol/g liver whereby its time course remained largely unchanged. With 1.25 mM Ca2+, UTP induced Ca2+ movements only near the detection level (i.e. below 10-20 nmol/g liver); however, with 0.3 mM Ca2+ in influent perfusate, there was a slow Ca2+ release (not completed within 5-6 min). The maximal rates of Ca2+ efflux following ATP and UTP (20 microM each) were 70 nmol and 30 nmol g-1 min-1. Withdrawal of UTP led to a short Ca2+ release superimposing a phase of net Ca2+ uptake. 6. The data show that extracellular UTP is a potential and effective regulator of hepatic metabolism, ion fluxes across the hepatocyte membrane and hemodynamics. Compared to ATP, UTP seems to be more effective and the responses to both nucleotides are different. The data suggest that the action of UTP could involve a receptor distinct from the purinergic P2 receptor, whereas the ATP action involves predominantly the P2Y purinoceptor subtype.

Perivascular nerve stimulation and phenylephrine responses in rat liver. Metabolic effects, Ca2+ and K+ fluxes

Electrical stimulation of perivascular nerves (20 Hz/2 ms/20 V) in perfused rat liver led to a transient increase of 14CO2 production from [1-14C]glutamate, glutathione and thiol efflux, an increase in the lactate/pyruvate and the 3-hydroxybutyrate/acetoacetate ratio, glucose release and of portal pressure. These metabolic effects were accompanied by a Ca2+ release from the liver within the initial 2 min, being followed by Ca2+ reuptake, which lasted about 3 min. The initial Ca2+ release was 67 nmol/g liver and was smaller than that observed after phenylephrine (5 microM) addition (156 nmol/g liver). Hepatic Ca2+ release following nerve stimulation or phenylephrine was not significantly affected when the hemodynamic changes were largely prevented by sodium nitroprusside (10 microM). Although the amounts of Ca2+ released were different, the glycogenolytic responses, but not the other metabolic effects, were quantitatively similar with nerve stimulation and phenylephrine. Within the first 3 min of nerve stimulation there was a K+ uptake by the liver being followed by a K+ release over the next 5-6 min and a subsequent slow K+ uptake phase. These changes resembled those observed with phenylephrine. Phentolamine, an alpha-adrenergic antagonist, abolished the Ca2+ and K+ movements following nerve stimulation as well as glucose release and the hemodynamic changes. During continuous infusion of phenylephrine, nerve stimulation led still to an increase of portal pressure; however, the effects of nerve stimulation on Ca2+ and K+ fluxes and glucose release were largely suppressed. It is concluded that the metabolic effects of electrical nerve stimulation are mediated by a redistribution of cellular Ca2+ following alpha-receptor activation. Nerve stimulation involves Ca2+ and K+ fluxes across the plasma membrane. The metabolic effects are qualitatively similar to those induced by phenylephrine. The quantitative difference between nerve stimulation and phenylephrine is explained by a differential subacinar response, with fewer cells being reached by nerve stimulation than cells containing alpha-receptors. The hemodynamic changes of nerve stimulation point to the existence of sphincters near the inflow of the sinusoidal bed.

Stimulation of glycogenolysis by adenine nucleotides in the perfused rat liver

Infusion of adenine nucleotides and adenosine into perfused rat livers resulted in stimulation of hepatic glycogenolysis, transient increases in the effluent perfusate [3-hydroxybutyrate]/[acetoacetate] ratio, and increased portal vein pressure. In livers perfused with buffer containing 50 microM-Ca2+, transient efflux of Ca2+ was seen on stimulation of the liver with adenine nucleotides or adenosine. ADP was the most potent of the nucleotides, stimulating glucose output at concentrations as low as 0.15 microM, with half-maximal stimulation at approx. 1 microM, and ATP was slightly less potent, half-maximal stimulation requiring 4 microM-ATP. AMP and adenosine were much less effective, doses giving half-maximal stimulation being 40 and 20 microM respectively. Non-hydrolysed ATP analogues were much less effective than ATP in promoting changes in hepatic metabolism. ITP, GTP and GDP caused similar changes in hepatic metabolism to ATP, but were 10-20 times less potent than ATP. In livers perfused at low (7 microM) Ca2+, infusion of phenylephrine before ATP desensitized hepatic responses to ATP. Repeated infusions of ATP in such low-Ca2+-perfused livers caused homologous desensitization of ATP responses, and also desensitized subsequent Ca2+-dependent responses to phenylephrine. A short infusion of Ca2+ (1.25 mM) after phenylephrine infusion restored subsequent responses to ATP, indicating that, during perfusion with buffer containing 7 microM-Ca2+, ATP and phenylephrine deplete the same pool of intracellular Ca2+, which can be rapidly replenished in the presence of extracellular Ca2+. Measurement of cyclic AMP in freeze-clamped liver tissue demonstrated that adenosine (150 microM) significantly increased hepatic cyclic AMP, whereas ATP (15 microM) was without effect. It is concluded that ATP and ADP stimulate hepatic glycogenolysis via P2-purinergic receptors, through a Ca2+-dependent mechanism similar to that in alpha-adrenergic stimulation of hepatic tissue. However, adenosine stimulates glycogenolysis via P1-purinoreceptors and/or uptake into the cell, at least partially through a mechanism involving increase in cyclic AMP. Further, the hepatic response to adenine nucleotides may be significant in regulating hepatic glucose output in physiological and pathophysiological states.

Effect of angiotensin II, ATP, and ionophore A23187 on potassium efflux in adrenal glomerulosa cells

Angiotensin II stimulus on perifused bovine adrenal glomerulosa cells elicited an increase in 86Rb efflux from cells previously equilibrated with the radioisotope. When 45Ca fluxes were measured under similar conditions, it was observed that Ca and Rb effluxes occurred within the first 30 s of the addition of the hormone and were independent of the presence of external Ca. The 86Rb efflux due to angiotensin II was inhibited by quinine and apamin. The hypothesis that the angiotensin II response is a consequence of an increase in the K permeability of the glomerulosa cell membrane triggered by an increase in cytosolic Ca is supported by the finding that the divalent cation ionophore A23187 also initiated 86Rb or K loss (as measured by an external K electrode). This increased K conductance was also seen with 10(-4) M ATP. Quinine and apamin greatly reduced the effect of ATP or A23187 on 86Rb or K release in adrenal glomerulosa cells. The results suggest that Ca-dependent K channels or carriers are present in the membranes of bovine adrenal glomerulosa cells and are sensitive to hormonal stimulus.

ATP-evoked membrane responses in Xenopus oocytes

Voltage-clamp technique and intracellular injections of drugs were used to study the adenosine triphosphate (ATP)-evoked depolarizing current response in the Xenopus laevis oocytes. The depolarizing current was comprised of a fast transient component (D1) followed by a late long-lasting component (D2). It was carried mainly by Cl- ions. The depolarizing current was better elicited by ATP and ADP than by AMP or adenosine and was not blocked either by theophylline (0.2 mM) or by quinidine sulphate (1 mM). The D2 current was sometimes masked by an ATP-evoked K+ hyperpolarizing current which was blocked by theophylline and mediated via P1 purinoceptors. This study suggests that the oocyte's membrane embodies at least two different purinoceptor's types, each of these types subserves a different set of ionic channels.

Effects of ATP and adenosine addition on activity of oxoglutarate dehydrogenase and the concentration of cytoplasmic free Ca2+ in rat hepatocytes

Addition of ATP (100 microM) to hepatocytes from starved rats incubated with 5 mM [1-14C]glutamine caused a stimulation of glucose formation; the magnitude of the concomitant increases in 14CO2 production and glutamine consumption indicate that flux from glutamine to glucose was increased. ATP also caused a simultaneous decrease in the cell content of oxoglutarate; together with the increased flux this is consistent with an activation of oxoglutarate dehydrogenase. In corroboration of this, a stimulation by ATP of gluconeogenesis and a decrease in oxoglutarate was also observed with 5 mM proline as substrate. ATP caused an increase in hepatocyte cytoplasmic free Ca2+ concentration, [Ca2+]c, as indicated by the increase in the fluorescence of cytoplasmically trapped quin2, from a resting value of about 0.2 microM to greater than 1 microM. The mechanism of oxoglutarate dehydrogenase activation may be via an increase in mitochondrial Ca2+ content as a consequence of the increase in [Ca2+]c. The effects of 100 microM adenosine were also investigated. An increase in flux from glutamine to glucose was observed together with a decrease in the cell oxoglutarate, thus indicating that adenosine addition to hepatocytes could also activate oxoglutarate dehydrogenase. The activation by adenosine was less than that produced by ATP. Adenosine caused a small apparent increase in [Ca2+]c to 0.3-0.4 microM; it remains to be established if this effect, which is small relative to that of ATP, is sufficient to elicit the activation of oxoglutarate dehydrogenase: alternative mechanisms may exist.

Regulation of canalicular bile formation by alpha-adrenergic action and by external ATP in the isolated perfused rat liver

In isolated perfused rat liver, addition of adrenaline induced a complex response of bile flow including rapid, reversible stimulation (1/2-2 min), reversible inhibition (2-10 min), and prolonged stimulation. Both the reversible stimulation and the inhibition were mimicked by the alpha-sympathomimetic agonist phenylephrine but not by the beta-agonist isoproterenol. The reversible stimulation was a very early effect being terminated prior to all other alpha-adrenergic responses of liver. External ATP considerably lowered bile flow while inducing release of glucose and lactate, inhibition of respiration, and a reversible efflux of Ca2+. Variations of mannitol clearance parallel to those of bile flow indicate a canalicular origin of all changes.

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+.

Ultrastructural and physiological effects of the ionophore A23187 at identified frog neuromuscular junctions

The physiological and morphological effects of the calcium ionophore A23187 (calimycin) at the frog neuromuscular junction in vitro were examined. Miniature endplate potentials were recorded intracellularly during exposure to the ionophore. Preparations fixed 15 or 30 min after adding the drug to the incubating medium, which exhibited a greatly increased miniature endplate potential frequency, showed no obvious morphological differences when compared to controls with regard to synaptic vesicle number or distribution of vesicles within the terminal. However, after 45-60 min of exposure to the ionophore, when miniature endplate potential frequency had declined almost to zero, most of the nerve endings appeared devoid of synaptic vesicles and other organelles while the plasma membrane was intact. It is suggested that the apparent depletion of vesicles from the terminal induced by the calcium ionophore is a consequence of irreversible changes at the terminal.

The contribution of both extracellular and intracellular calcium to the action of alpha-adrenergic agonists in perfused rat liver

The role of both intracellular and extracellular Ca2+ pools in the expression of alpha-adrenergic-agonist-mediated responses was examined in perfused rat liver. Responses studied included glycogenolysis, respiration, lactate and pyruvate formation, ketone-body production, changes in the cytoplasmic and mitochondrial redox ratio and cellular K+ fluxes. Each of these was shown to be dependent on the mobilization of intracellular Ca2+ and can be grouped into one of two response types. Transient responses (ion fluxes and the redox ratios) are obligatorily dependent on the mobilization of intracellular Ca2+ and occur irrespective of the extracellular Ca2+ concentration. Sustained responses, on the other hand, initially require intracellular Ca2+ and, subsequently, extracellular Ca2+. The data indicate that alpha-adrenergic agonists mobilize extracellular Ca2+ as well as intracellular Ca2+ and that both pools are required for the full expression of hormone-induced responses in rat liver.

The second messenger linking receptor activation to internal Ca release in liver

The increase in cytosolic [Ca2+] induced by Ca-mobilizing hormones in liver is mainly due to release of Ca from intracellular stores. For Ca to be released from internal sites a messenger must be formed at the plasma membrane which diffuses into the cytosol to signal Ca release from the intracellular organelles. One of the first actions of these hormones is to cause breakdown of the polyphosphoinositides to form soluble inositol phosphates. Some evidence for the idea that these substances could be the second messenger has been obtained in pancreatic acinar cells. Here we have found that hormone activation of hepatocytes causes rapid breakdown of phosphatidylinositol 4,5-bisphosphate [ PtdIns (4,5)P2] to form inositol trisphosphate ( InsP3 ). When applied to permeabilized hepatocytes, InsP3 releases Ca from non-mitochondrial ATP-dependent pools. This suggests that InsP3 could be the messenger linking Ca-mobilizing receptor activation to intracellular Ca release in liver.

Induction of taurocholate release from isolated rat hepatocytes in suspension by alpha-adrenergic agents and vasopressin: implications for control of bile salt secretion

Hepatocytes incubated with 25 muM [3H] taurocholate rapidly deplete the extracellular medium of [3H] taurocholate and achieve a steady-state level of intracellular bile salt within 15 min. Exposure of cells at steady state with extracellular taurocholate to the catecholamines norepinephrine or epinephrine results in release of 3H from the cells into the incubation medium; the 3H released represents almost exclusively unmetabolized [3H] taurocholate. The hierarchy of effectiveness of the catecholamines, norepinephrine congruent to epinephrine greater than phenylephrine much greater than isoproterenol, is indicative of an alpha-adrenergic mechanism. Induction of [3H] taurocholate release by norepinephrine is inhibited by the alpha-antagonists phenoxybenzamine and phentolamine and by chlorpromazine, but is not affected by the beta-antagonist propranolol, further supporting an alpha-adrenergic basis for this phenomenon. Arginine vasopressin, at concentrations of 1 X 10(-9) M and greater, also induces bile salt release. Classical alpha- and beta-antagonists have minimal effects on vasopressin induced bile salt release. While the peptide hormones angiotensin and oxytocin are, alone, relatively ineffective inducers of bile salt release, oxytocin potentiates the induction of bile salt release by vasopressin, suggesting complex interactions with membrane receptor function. Further studies assessing the interaction of sympathetic neurotransmitters and peptide hormones with bile salt transport and release in the hepatocyte may provide insight into the regulation of hepatic secretory function in the intact animal.

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.

The effect of ionophore A23187 on albumin internalization in cultured human umbilical vein endothelial cells

The effects of calcium and the calcium ionophore A23187 on endocytosis were studied in cultured human umbilical vein endothelial cells using iodinated human albumin to measure bulk phase endocytosis. In the absence of the ionophore, varying the levels of extracellular calcium did not affect endocytosis. In the presence of 10 microM A23187, the endocytic clearance of albumin decreased approx. 50% when exposed to physiological concentrations of extracellular calcium, but increased approx. 50% at lower calcium concentrations. Since the ionophore is known to alter cellular calcium levels, these results are compatible with a role for intracellular calcium in the modulation of endothelial cell endocytosis.

Factors influencing calcium-induced terminal differentiation in cultured mouse epidermal cells

Mouse epidermal cells can be grown as a proliferating monolayer in medium containing 0.02-0.1 mM calcium. Terminal differentiation of these cells with formation of cornified cells and cell death is induced by elevating calcium in the medium to greater than 0.1 mM. A variety of agents were studied as potential modifiers of this calcium-induced terminal differentiation. Other than calcium, no cation tested was active in inducing or preventing epidermal maturation. Modifiers of calcium or sodium fluxes, local anesthetics and protease inhibitors were also without effect. Modulators or analogues of cyclic nucleotides did not influence epidermal differentiation, and cyclic nucleotide levels did not change significantly in the first 10 min after increasing calcium. Effective inhibition of calcium-induced differentiation, as estimated by morphology, ultrastructure and cornified envelope formation, was seen with the divalent cation ionophore A23187 and the Na+K+ATPase inhibitor ouabain. The well-known effects of ouabain on intracellular sodium and potassium suggested the possible involvement of these ions in the program of calcium-induced epidermal maturation. The increase in medium calcium produced an elevation of both intracellular sodium and potassium within 12-24 hours. The calcium-induced increase in intracellular potassium appears to be the more relevant of these changes since the increase was blocked by both ouabain and A23187. Other inhibitors of calcium-induced differentiation, including harmaline, 8(diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8) and low potassium medium, also blocked the rise of intracellular potassium. The five inhibitors had no consistent effect on intracellular sodium. Thus, elevated intracellular potassium may be necessary for the later stages of epidermal differentiation. However, neither ouabain nor A23187 affected the assembly of desmosomes, the earliest ultrastructural change noted after increasing medium calcium. This rapid change in cell-cell contact, beginning within minutes after calcium elevation, appears to be independent of changes in sodium and potassium, but may instead be modulated by increased calcium at the cell surface.

The effect of ionophore A23187 on calcium ion fluxes and alpha-adrenergic-agonist action in perfused rat liver

The effect of ionophore A23187 on cellular Ca2+ fluxes, glycogenolysis and respiration was examined in perfused liver. At low extracellular Ca2+ concentrations (less than 4 microM), A23187 induced the mobilization of intracellular Ca2+ and stimulated the rate of glycogenolysis and respiration. As the extracellular Ca2+ concentration was elevated, biphasic cellular Ca2+ fluxes were observed, with Ca2+ uptake preceding Ca2+ efflux. Under these conditions, both the glycogenolytic response and the respiratory response also became biphasic, allowing the differentiation between the effects of extracellular and intracellular Ca2+. Under all conditions examined the rate of Ca2+ efflux induced by A23187 was much slower than the rate of phenylephrine-induced Ca2+ efflux, although the net amounts of Ca2+ effluxed were similar for both agents. The effect of A23187 on phenylephrine-induced Ca2+ fluxes, glycogenolysis and respiration is dependent on the extracellular Ca2+ concentration. At concentrations of less than 50 microM-Ca2+, A23187 only partially inhibited alpha-agonist action, whereas at 1.3 mM-Ca2+ almost total inhibition was observed. The action of A23187 at the cellular level is complex, dependent on the experimental conditions used, and shows both differences from and similarities to the hepatic action of alpha-adrenergic agonists.

On the mechanisms of ATP-induced and succinate-induced redistribution of cations in isolated rat liver cells

1. The ability of external ATP to induce calcium uptake in isolated rat liver cells was further characterized. Stimulation of calcium uptake was specific for ATP, other nucleotides or ATP metabolites had no comparable effect. ATP was dephosphorylated while stimulating calcium uptake, but there was no stoichiometry between ATP hydrolysis and calcium uptake nor did dephosphorylation depend on calcium concentration. ATP acted from outside and was dephosphorylated by an ecto-ATPase of the cells. 2. In addition to its direct action, ATP enhanced succinate-dependent calcium uptake in a cooperative fashion. This is best explained by different sites of action. ATP increases cell membrane permeability while succinate stimulates uptake into mitochondria. 3. ATP was able to lower Na+ and K+ gradients and the pH gradient between cells and incubation medium. Increasing calcium concentration counteracted this effect though calcium uptake was then stimulated. 4. Succinate alone did not affect monovalent cation gradients but raised the pH gradient. It partially counteracted the ATP effects on these gradients. 5. Since catecholamine-like actions of ATP may be mediated by an increase in cytoplasmic calcium concentration, the action of extracellular ATP can be taken as a model to study the role of calcium as a transmitter of hormone actions. From interdependence between ATP-stimulated and succinate-stimulated calcium uptake, conclusions can be drawn on the resulting cytoplasmic calcium concentration and its effect on plasma membrane permeability.

alpha-adrenergic stimulation of respiration in isolated rat hepatocytes

1. The alpha-adrenergic agonists noradrenaline (in the presence of beta-blocker) and phenylephrine cause a transient stimulation of the respiration in isolated rat hepatocytes. After a lag period of 12s, this activation first attains its maximal value (+24%) for about 1 min and then falls to a sustained value (+15%). The effect is blocked by the alpha-antagonists phenoxybenzamine and phentolamine. It is dose-dependent, with an half-maximal stimulation by 16 nM-noradrenaline, which is similar to that found for other cell responses to the hormone. 2. Vasopressin and ATP, which in common with alpha-agonists are believed to increase intracellular [Ca2+], induce similar activation in the respiration rate. 3. The alpha-adrenergic-mediated respiration depends on extracellular Ca2+. The activation is decreased or abolished when extracellular [Ca2+] is decreased by adding EGTA, or when the Ca2+ antagonists Mn2+ and La3+ are present in the incubation medium. 4. It is suggested that the activation of the mitochondrial respiration rate results from the increase in cytosolic Ca2+ concentration, presumably via Ca2+ influx or Ca2+ release from the plasma membrane or endoplasmic reticulum.

The effect of alpha-adrenergic agonists on the membrane potential of fat-cell mitochondria in situ

1. The accumulation of [3H]methyltriphenylphosphonium by isolated fat-cells was used to estimate the membrane potential of mitochondria in situ. 2. An alpha-adrenergic receptor-mediated decrease in the apparent accumulation of [3H]methyltriphenylphosphonium was observed. Methoxamine, clonidine and low concentrations of phenylephrine decreased the calculated mitochrondrial membrane potential without significantly raising cyclic AMP levels, adenylate cyclase activity or stimulating lipolysis. The agonist potency order was phenylephrine greater than methoxamine greater than clonidine. 3. The decrease in the calculated mitochondrial membrane potential caused by phenylephrine, clonidine and methoxamine was blocked by the alpha-adrenergic antagonist prazosin but not by yohimbine nor by the beta-antagonist propranolol. This suggests that the effect on the calculated mitochondrial membrane potential may be mediated by alpha 1-like receptors.

Stimulation of Na+ -dependent amino acid uptake by activation of the Ca2+ -dependent K+ channel in the Ehrlich ascites tumor cell

The activation of Ca2+ -dependent K+ channel by propranolol or by ascorbate-phenazine methosulphate stimulates Na+ -dependent transport of alpha-aminoisobutyric acid. This stimulation arises from a membrane hyperpolarization due to the specific increase of membrane K+ conductance. The same treatment does not modify the Na+ -independent uptake of the norbornane amino acid.

Ca2+-dependent K+ transport in the Ehrlich ascites tumor cell

The possible presence and properties of the Ca2+-dependent K+ channel have been investigated in the Ehrlich ascites tumor cell. The treatment with ionophore A23187 + CA2+, propranolol or the electron donor system ascorbate-phenazine methosulphate, all of which activate that transport system in the human erythrocyte, produces in the Ehrlich cell a net loss of K+ (balanced by the uptake of Na+) and a stimulation of both the influx and the efflux of 86Rb. These effects were antagonized by quinine, a known inhibitor of the Ca2+-dependent K+ channel in other cell systems, and by the addition of EGTA to the incubation medium. Ouabain did not have an inhibitory effect. These results suggests that the Ehrlich cell possesses a Ca2+-dependent K+ channel whose characteristics are similar to those described in other cell systems.

Cell-membrane receptors for purines. Review

Purines are involved in many aspects of cell chemistry - intermediary metabolism, nucleic acid synthesis, and the supply of high-energy phosphates to various active transport systems. In addition, however, there appear to be specific receptor molecules located within the plasma membrane of some cells, which mediate changes of cell function in response to purines present in the extracellular fluid. It is the purpose of this review to summarize the kind of functions subserved by those receptors as well as the basic structural requirements for their activation.