Biomedical subjects

J W Putney

Publications and source records attributed to J W Putney.

At least 109 records · Page 6Linked to original sources

PubMed1988

Metabolism and functions of inositol phosphates.

Activation of Ca2+-mobilizing receptors rapidly increases the cytoplasmic Ca2+ concentration both by releasing Ca2+ stored in endoplasmic reticulum and by stimulating Ca2+ entry into the cells. The mechanism by which Ca2+ release occurs has recently been elucidated. Receptor activation of phospholipase C results in the hydrolysis of the plasma membrane lipid, phosphatidylinositol 4,5-bisphosphate (PIP2), to yield two intracellular messengers, diacylglycerol (DAG) and (1,4,5)inositol trisphosphate [(1,4,5)IP3]. DAG remains in the plasma membrane where it stimulates protein phosphorylation via the phospholipid-dependent protein kinase C. (1,4,5)IP3 diffuses to and interacts with specific sites on the endoplasmic reticulum to release stored Ca2+. Receptor stimulation of phospholipase C appears to be mediated by one or more guanine nucleotide-dependent regulatory proteins by a mechanism analogous to hormonal activation of adenylyl cyclase. The actions of (1,4,5)IP3 on Ca2+ mobilization are terminated by two metabolic pathways, sequential dephosphorylation to inositol bisphosphate (IP2), inositol monophosphate (IP) and inositol or by phosphorylation to inositol tetrakisphosphate (IP4) and sequential dephosphorylation to different inositol phosphates. A sustained cellular response also requires Ca2+ entry into the cell from the extracellular space. The mechanism by which hormones increase Ca2+ entry is not known; a recent proposal involving movement of Ca2+ through the endoplasmic reticulum, possibly regulated by IP4, will be considered here.

Animals↗

PubMed1987

Homologous desensitization of substance-P-induced inositol polyphosphate formation in rat parotid acinar cells.

Maximal concentrations of substance P and methacholine induced a rapid increase in [3H]inositol trisphosphate ([3H]IP3) formation. After about 1 min, the [3H]IP3 in the substance-P-treated cells ceased to increase further, whereas in the methacholine-treated cells [3H]IP3 continued to increase. Addition of methacholine to the substance-P-treated cells caused a rapid increase in [3H]IP3, whereas a second addition of a 10-fold excess of substance P had no effect. Pretreatment of cells with substance P, followed by removal of the substance P by washing, resulted in a decreased response to a second application of substance P. A similar protocol involving pretreatment with methacholine had no effect on subsequent responsiveness to substance P. Analysis of [3H]substance P binding to substance-P-treated cells indicated that the number of receptors for substance P was decreased, but the affinity of the receptors for substance P was unaffected. After substance P pretreatment, a prolonged incubation (2 h) restored responsiveness of the cells to substance P, measured as [3H]IP3 formation, and restored the number of binding sites to control values. These findings indicate that, in the rat parotid gland, substance P induces a homologous desensitization of its receptor, which involves a slowly reversible down-regulation or sequestration of substance-P-binding sites.

Animals↗

PubMed1987

Metabolism of inositol 1,4,5-trisphosphate in guinea-pig hepatocytes.

Metabolism of inositol 1,4,5-trisphosphate was investigated in permeabilized guinea-pig hepatocytes. The conversion of [3H]inositol 1,4,5-trisphosphate to a more polar 3H-labelled compound occurred rapidly and was detected as early as 5 s. This material co-eluted from h.p.l.c. with inositol 1,3,4,5 tetrakis[32P]phosphate and is presumably an inositol tetrakisphosphate. A significant increase in the 3H-labelled material co-eluting from h.p.l.c. with inositol 1,3,4-trisphosphate occurred only after a definite lag period. Incubation of permeabilized hepatocytes with inositol 1,3,4,5-tetrakis[32P]phosphate resulted in the formation of 32P-labelled material that co-eluted with inositol 1,3,4-trisphosphate; no inositol 1,4,5-tris[32P]phosphate was produced, suggesting the action of a 5-phosphomonoesterase. The half-time of hydrolysis of inositol 1,3,4,5-tetrakis[32P]phosphate of approx. 1 min was increased to 3 min by 2,3-bisphosphoglyceric acid. Similarly, the rate of production of material tentatively designed as inositol 1,3,4-tris[32P]phosphate from the tetrakisphosphate was reduced by 10 mM-2,3-bisphosphoglyceric acid. In the absence of ATP there was no conversion of [3H]inositol 1,4,5-trisphosphate to [3H]inositol tetrakisphosphate or to [3H]inositol 1,3,4-trisphosphate, which suggests that the 1,3,4 isomer does not result from isomerization of inositol 1,4,5-trisphosphate. The results of this study suggest that the origin of the 1,3,4 isomer of inositol trisphosphate in isolated hepatocytes is inositol 1,3,4,5-tetrakisphosphate and that inositol 1,4,5-trisphosphate is rapidly converted to this tetrakisphosphate. The ability of 2,3-bisphosphoglyceric acid, an inhibitor of 5-phosphomonoesterase of red blood cell membrane, to inhibit the breakdown of the tetrakisphosphate suggests that the enzyme which removes the 5-phosphate from inositol 1,4,5-trisphosphate may also act to convert the tetrakisphosphate to inositol 1,3,4-trisphosphate. It is not known if the role of inositol 1,4,5-trisphosphate kinase is to inactivate inositol 1,4,5-trisphosphate or whether the tetrakisphosphate product may have a messenger function in the cell.

2,3-Diphosphoglycerate↗

PubMed1987

The B lymphocyte calcium response to anti-Ig is diminished by membrane immunoglobulin cross-linkage to the Fc gamma receptor.

We report the cytosolic free calcium, [Ca2+]i, responses of single murine B lymphocytes to whole and F(ab')2 fragments of anti-Ig measured in the flow cytometer with indo-1, a new fluorescent chelator of calcium. The principle advantages of this recording system are these: Indo-1 is highly fluorescent; hence, loading concentrations that introduce artifacts in the reported [Ca2+]i signal may be avoided. The measurement of [Ca2+]i by fluorescence ratio corrects for nonuniform dye uptake, making possible quantitative estimates of [Ca2+]i in single cells and an assessment of the variability of population responses. Baseline recordings of unstimulated lymphocytes indicated a narrow, stable range of [Ca2+]i (75 to 125 nM). The [Ca2+]i rise induced by various anti-Ig preparations exhibited considerable heterogeneity. The initial mean value for F(ab')2 anti-Ig-stimulated cells peaked above 1 microM and was due only to the release of Ca2+ from intracellular stores. A steady state elevation of [Ca2+]i was reached by 5 min and persisted for hours. Cells stimulated with intact anti-Ig reached similar initial peak [Ca2+]i values, but then declined toward baseline. This difference was due to membrane Ig-IgG Fc receptor (mIg-Fc gamma R) cross-linkage, because blocking the Fc gamma R with a monoclonal antibody made the [Ca2+]i responses to F(ab')2 and intact anti-Ig identical. The attenuation of the [Ca2+]i signal by mIg-Fc gamma R cross-linkage is proceeded by a corresponding Fc gamma-mediated reduction in anti-Ig-induced inositol trisphosphate elevation. These findings outline a biochemical basis for mIg- and Fc gamma R-mediated activation and regulation intrinsic to the B cell, and demonstrate the advantages of indo-1 over quin2 for fluorescent measurement of [Ca2+]i in small cells.

Animals↗

PubMed1987

Formation and actions of calcium-mobilizing messenger, inositol 1,4,5-trisphosphate.

A variety of surface membrane receptors can activate a phospholipase C, which degrades phosphatidylinositol 4,5-bisphosphate liberating a calcium mobilizing second messenger, inositol 1,4,5-trisphosphate [(1,4,5)IP3]. The coupling of surface receptors to the phospholipase C involves one or more guanine nucleotide-dependent regulatory proteins that are similar but not identical to those that regulate adenylate cyclase. (1,4,5)IP3 has been shown to release Ca2+ from a portion of the endoplasmic reticulum and is believed responsible for the initial phase of Ca2+ mobilization ascribed to internal Ca2+ release. (1,4,5)IP3 acts by binding to a specific receptor that either is a component of, or regulates, a Ca2+ ion channel. The release of Ca2+ from the (1,4,5)IP3-sensitive component of the endoplasmic reticulum may secondarily activate the second phase of Ca2+ mobilization, which involves Ca2+ entry. (1,4,5)IP3 is metabolized by two pathways. One involves the action of a 5-phosphatase that degrades (1,4,5)IP3 to inositol 1,4-bisphosphate, whereas the other involves a 3-kinase that phosphorylates (1,4,5)IP3 to produce inositol 1,3,4,5-tetrakisphosphate. The significance of this dual metabolism is not known, but it may be important in rapidly extinguishing the Ca2+-releasing activity (1,4,5)IP3.

Animals↗

PubMed1987

Mobilization of intracellular calcium by methacholine and inositol 1,4,5-trisphosphate in rat parotid acinar cells.

In the rat parotid acinar cell, methacholine caused an increase in [Ca2+]i as determined by quin-2 fluorescence. The increase in [Ca2+]i was initially independent of, and subsequently dependent on, the presence of extracellular Ca2+, indicating mobilization of intracellular Ca2+, as well as activation of Ca2+ entry. Methacholine mobilization of the internal Ca2+ pool and stimulation of the initial transient phase of K+ efflux have similar concentration dependencies; the EC50 value for Ca2+ mobilization is 80 nmol/L, the EC50 value for K+ efflux is 200 nmol/L. In a permeable parotid cell preparation, inositol 1,4,5-trisphosphate, inositol 2,4,5-trisphosphate, and inositol 4,5-bisphosphate were able to release Ca2+ from an ATP-dependent, oligomycin-insensitive pool. These observations, when taken with the previous finding that methacholine stimulates Ca-independent inositol trisphosphate formation, support the view that inositol 1,4,5-trisphosphate acts as a second messenger mediating the release of an intracellular Ca2+ pool following muscarinic receptor activation in the parotid gland.

Aminoquinolines↗

PubMed1986

Stoichiometry of contraction and Ca2+ mobilization by inositol 1,4,5-trisphosphate in isolated gastric smooth muscle cells.

Smooth muscle cells were isolated from the circular muscle layer of guinea pig stomach and permeabilized by brief exposure to saponin. Both permeabilized and intact muscle cells contracted in response to cholecystokinin octapeptide (CCK-8) and acetylcholine, but only permeabilized muscle cells contracted in response to inositol 1,4,5-trisphosphate (InsP3). The contractile response to InsP3 was prompt (peak less than 5 s), concentration-dependent (EC50-0.3 microM), and insensitive to antimycin or oligomycin. Contraction induced by either InsP3 or CCK-8 was accompanied by a concentration-dependent increase in free Ca2+ that was directly correlated with the magnitude of contraction. Both InsP3 and CCK-8 caused rapid net efflux of Ca2+ from cells preloaded with 45Ca2+. Contraction, increase in free Ca2+ concentration, and net 45Ca2+ efflux elicited by a combination of maximal concentrations of InsP3 and CCK-8 were not significantly different from those elicited by maximal concentrations of either agent alone. Repeated stimulation of single muscle cells with either InsP3 or CCK-8 in Ca2+-free medium caused eventual loss of the contractile response to all agents. The response to all agents was restored upon re-exposure of the cell to a cytosol-like concentration of Ca2+, implying equal access of InsP3 and receptor-linked agonists to the same intracellular Ca2+ store. The results demonstrate that InsP3 mimics the effects of receptor-linked agonists on contraction and mobilization of intracellular Ca2+ in permeabilized smooth muscle cells that retain the functional properties of intact smooth muscle cells and support a role for InsP3 as membrane-derived messenger responsible for mobilization of intracellular Ca2+ in smooth muscle cells.

Animals↗

PubMed1986

Effects of Ca2+ on phosphoinositide breakdown in exocrine pancreas.

Recent studies have established that inositol 1,4,5-trisphosphate [I(1,4,5)P3] provides the link between receptor-regulated polyphosphoinositide hydrolysis and mobilization of intracellular Ca2+. Here, we report the effects of Ca2+ on inositol trisphosphate (IP3) formation from phosphatidylinositol bisphosphate (PIP2) catalysed by phospholipase C in intact and electrically permeabilized rat pancreatic acinar cells. In permeabilized cells, the Ca2+-mobilizing agonist caerulein stimulated [3H]IP3 formation when the free [Ca2+] was buffered at 140 nM, the cytosolic free [Ca2+] of unstimulated pancreatic acinar cells. When the free [Ca2+] was reduced to less than 10 nM, caerulein did not stimulate [3H]IP3 formation. Ca2+ in the physiological range stimulated [3H]IP3 formation and reduced the amount of [3H]PIP2 in permeabilized cells. The effects of Ca2+ and the receptor agonist caerulein were additive, but we have not established whether this reflects independent effects on the same or different enzymes. The effect of Ca2+ on [3H]IP3 formation by permeabilized cells was unaffected by inhibitors of the cyclo-oxygenase and lipoxygenase pathways of arachidonic acid metabolism; nor were the effects of Ca2+ mimicked by addition of arachidonic acid. These results suggest that the effects of Ca2+ on phospholipase C activity are not a secondary consequence of Ca2+ activation of phospholipase A2. Changes in free [Ca2+] (less than 10 nM-1.2 mM) did not affect the metabolism of exogenous [3H]I(1,4,5)P3 by permeabilized cells. In permeabilized cells, breakdown of exogenous [3H]IP3 to [3H]IP2 (inositol bisphosphate), and formation of [3H]IP3 in response to receptor agonists were equally inhibited by 2,3-bisphosphoglyceric acid. This suggests that the [3H]IP2 formed in response to receptor agonists is entirely derived from [3H]IP3. In intact cells, [3H]IP3 formation was stimulated when ionomycin was used to increase the cytosolic free [Ca2+]. However, a maximal concentration of caerulein elicited ten times as much IP3 formation as did the highest physiologically relevant [Ca2+]. We conclude that the major effect of receptor agonists on IP3 formation does not require an elevation of cytosolic free [Ca2+], although the increase in free [Ca2+] that normally follows IP3 formation may itself have a small stimulatory effect on phospholipase C.

Animals↗

PubMed1986

Isomers of inositol trisphosphate in exocrine pancreas.

In rat pancreatic acinar cells, the Ca2+-mobilizing receptor-agonist, caerulein, at both maximal and submaximal concentrations, stimulated a rapid, transient, increase in [3H]inositol 1,4,5-trisphosphate [(1,4,5)IP3], followed by a slower, sustained, increase in [3H]inositol 1,3,4-trisphosphate [(1,3,4)IP3]. Neither activation of protein kinase C by phorbol dibutyrate nor prevention of the caerulein-stimulated elevation of cytosolic [Ca2+] significantly affected the pattern of formation of the two isomers of IP3. Although carbachol evoked an increase in cytosolic [Ca2+], it did not significantly stimulate [3H](1,4,5)IP3 accumulation, but did promote [3H](1,3,4)IP3 accumulation. Moreover, both carbachol and caerulein maintained hormone-sensitive intracellular Ca2+ pools in a Ca2+-depleted state after [3H](1,4,5)IP3 had returned to basal concentrations. One interpretation of these findings is that total cellular concentrations of [3H](1,4,5)IP3 may not accurately reflect the concentration of this putative mediator in biologically relevant compartments.

Animals↗

PubMed1986

ATP-induced calcium mobilization and inositol 1,4,5-triphosphate formation in H-35 hepatoma cells.

Addition of ATP (but not epinephrine, angiotensin II, vasopressin, or platelet-activating factor) to H-35 hepatoma cells whose cellular lipids have been pre-labelled with [3H]inositol, causes a rapid increase in [3H]inositol triphosphate. In H-35 cells pre-incubated in the presence of 45Ca2+, ATP causes a similarly rapid release of 45Ca2+. The concentration-effect relationships for inositol triphosphate formation and Ca2+ efflux are similar to those reported previously for differentiated hepatocytes. These results demonstrate that at least one of the Ca2+-mobilizing receptors normally found on hepatocytes is functionally retained in the H-35 hepatoma cell line and thus could provide a useful model for the study of these receptor mechanisms in liver.

Adenosine Triphosphate↗

PubMed1986

Evidence suggesting that a novel guanine nucleotide regulatory protein couples receptors to phospholipase C in exocrine pancreas.

The initial response of many cells to 'Ca2+-mobilizing' agonists is phospholipase C-mediated hydrolysis of phosphatidylinositol bisphosphate to inositol trisphosphate (IP3) and diacylglycerol. It has been suggested, by analogy with receptor regulation of adenylate cyclase, that 'Ca2+-mobilizing' receptors may interact with a guanine nucleotide-binding protein (G protein) to regulate phospholipase C activity. Here we report increased accumulation of IP3 in response to caerulein or carbachol in electrically permeabilized rat pancreatic acinar cells. The stable analogues of GTP (guanosine 5'-[gamma-thio]trisphosphate and guanosine 5'-[beta, gamma-imido]triphosphate) stimulate IP3 accumulation and potentiate the effects of caerulein and carbachol. This synergism demonstrates an interaction between receptors, a G protein and phospholipase C. These responses are unaffected by pretreatment of the cells with pertussis or cholera toxins under conditions that produce substantial covalent modification of Gi and Gs, the proteins that couple receptors to adenylate cyclase. We therefore conclude that the G protein that couples receptors to phospholipase C in exocrine pancreas is probably neither Gi nor Gs; instead, we propose that a different G protein mediates this effect.

Adenylate Cyclase Toxin↗

PubMed1986

Cytosolic calcium during contraction of isolated mammalian gastric muscle cells.

During activation of visceral smooth muscle there is an increase in cytosolic-free calcium, but the source (intracellular calcium release or calcium influx), kinetics, and stoichiometry of this increase have not been determined. Here, the fluorescent indicator, quin2-acetoxymethyl ester, was used to measure directly cytosolic-free calcium during contraction of isolated stomach muscle cells induced by the two neuropeptides cholecystokinin-octapeptide and Met-enkephalin as well as acetylcholine. An increase in cytosolic-free calcium was seen that was (i) dependent on the concentration of contractile agonist, (ii) derived from intracellular sources (that is, not significantly affected by removal of ambient calcium or addition of a calcium channel blocker), and (iii) kinetically and stoichiometrically related to net calcium efflux and contraction. In contrast, the increase in cytosolic-free calcium induced by depolarizing concentrations of potassium was caused by influx of calcium through voltage-dependent calcium channels.

Acetylcholine↗

PubMed1986

A guanine nucleotide-dependent regulatory protein couples substance P receptors to phospholipase C in rat parotid gland.

Electrically permeabilized cells of rat parotid gland, prelabelled with [3H]-inositol, synthesized [3H]-inositol phosphates (IP3 and IP2) when stimulated with alpha 1-adrenergic, muscarinic-cholinergic, and substance P receptor-agonists. Non-hydrolyzable analogues of GTP (GTP gamma S and GppNHp) also stimulated [3H]-IP3 formation by permeabilized cells and they potentiated the stimulation by receptor-agonists. These effects of guanine nucleotides occurred only with GTP analogues and only in permeabilized cells indicating an intracellular site of action. NaF stimulated [3H]-IP3 accumulation, an effect that was not entirely attributable to the ability of F- to inhibit (1,4,5)IP3 degradation. These results suggest that a guanine nucleotide-dependent regulatory protein couples Ca2+-mobilizing receptors to phospholipase C in parotid gland.

Animals↗

PubMed1986

A model for receptor-regulated calcium entry.

A model is proposed for the mechanism by which activation of surface membrane receptors causes sustained Ca2+ entry into cells from the extracellular space. Reassessment of previously published findings on the behavior of receptor-regulated intracellular Ca2+ pools leads to the conclusion that when such pools are empty, a pathway from the extracellular space to the pool is opened; conversely when the pool is filled, the pathway is closed and it becomes relatively stable to depletion by low Ca2+ media or chelating agents. The biphasic nature of agonist-activated Ca2+-mobilization is thus seen as an initial emptying of the intracellular Ca2+ pool by inositol (1,4,5) trisphosphate, followed by rapid entry of Ca2+ into the pool and, in the continued presence of inositol (1,4,5) trisphosphate, into the cytosol. On withdrawal of agonist, inositol (1,4,5) trisphosphate is then rapidly degraded, the pathway from the pool to the cytosol is closed, and rapid entry from the outside continues until the Ca2+ content of the pool reaches a level that inactivates Ca2+ entry. This capacitative model allows for Ca2+ release and Ca2+ entry to be controlled by a single messenger, inositol (1,4,5) trisphosphate.

Animals↗

PubMed1986

Identification of cellular activation mechanisms associated with salivary secretion.

In recent years, our understanding of receptor-signalling mechanisms in the salivary glands has advanced considerably. Two receptor pathways exist, one involving cAMP, which primarily regulates enzyme secretion, and another involving the hydrolysis of PIP2, which regulates Ca2+ mobilization and, subsequently, monovalent ion fluxes probably important in ion and water secretion in the intact gland. Mobilization of Ca2+ results from both the release of internal Ca2+, and from Ca2+ entry from the extracellular space. The signal for Ca2+ release appears to be (1,4,5)IP3, one of the water soluble products of PIP2 hydrolysis. The mechanism controlling Ca entry is not understood, but speculation abounds. Hydrolysis of PIP2 also produces DG, which has a messenger role in activating a specific protein kinase, the C kinase. The C kinase interacts with Ca2+ mobilization in some as yet uncharacterized way in regulating enzyme secretion.

Adenosine Triphosphate↗

PubMed1986

Source of activator calcium in isolated guinea pig and human gastric muscle cells.

The source of Ca2+ responsible for contraction was examined in suspensions of smooth muscle cells and in perfused single muscle cells from guinea pig and human stomach. In both preparations removal of Ca2+ from the medium or addition of the Ca2+ channel blocker methoxyverapamil had no effect on the contractile response to various agonists, including cholecystokinin octapeptide (CCK-8) and acetylcholine, but inhibited the response to high extracellular K+ by 76-82%. Repeated stimulation of guinea pig or human single muscle cells in Ca2+-free medium, or in the presence of methoxyverapamil caused a progressive decrease and eventual abolition of contractile response; response was restored on restitution of Ca2+ to the medium or elimination of methoxyverapamil. Measurement of 45Ca2+ content in guinea pig muscle cells showed that CCK-8 had no effect on the rate of Ca2+ influx but increased the rate of Ca2+ efflux transiently by sixfold. Net peak efflux coincided with the time of peak contraction and was stoichiometrically related to the degree of contraction. Equipotent, maximally effective contractile doses of CCK-8, acetylcholine, and methionine-enkephalin caused equivalent degrees of net Ca2+ efflux. The results indicate that contractile agonists cause release of Ca2+ from a depletable intracellular store in gastric muscle cells. The release is accompanied by a dose-dependent increase in Ca2+ efflux and is capable of sustaining an initial maximal contraction. Repeated contractile activity requires influx of Ca2+ from extracellular sources.

Acetylcholine↗

PubMed1986

Formation and biological action of inositol 1,4,5-trisphosphate.

A wide variety of receptors appear to be coupled to a phospholipase C (EC 3.1.4.3) that hydrolyzes inositol lipids. This reaction is believed to provide a link between receptor activation and cellular Ca2+ mobilization. The mechanisms by which this occurs are believed to involve inositol 1,4,5-trisphosphate (1,4,5-IP3), which signals release of Ca2+ from the endoplasmic reticulum. In rat parotid acinar cells made permeable with saponin, 1,4,5-IP3 induced rapid release of sequestered Ca2+. In intact parotid cells, the concentration-response relationship for methacholine-induced IP3 formation was similar to the relationship for muscarinic receptor occupancy by methacholine. About 10-fold lower concentrations of methacholine were sufficient to increase cytosolic [Ca2+] and to activate secretion, indicating an excess IP3 forming capacity for the muscarinic receptor. The mechanisms for the coupling of receptors to IP3 formation were studied in pancreatic acinar cells made permeable electrically. In this preparation, nonhydrolyzable derivatives of GTP potentiated agonist-induced IP3 production, which suggests the involvement of a guanine nucleotide-dependent regulatory protein. The effects of agonists and guanine nucleotides were not altered by pretreating the acinar cells with cholera or pertussis toxins, which indicated that the regulatory protein linking receptors to IP3 formation is distinct from the ones involved in the regulation of adenylate cyclase.

Animals↗

PubMed1986

Further studies on the interactions between the calcium mobilization and cyclic AMP pathways in guinea pig hepatocytes.

Isoproterenol (50 nM) potentiated the effects of angiotensin (1-50 nM) on 86Rb efflux and 45Ca efflux from guinea pig hepatocytes. This effect occurred in the presence or absence of extracellular Ca2+ and required the simultaneous presence of both isoproterenol and angiotensin. Neither the divalent cationophore, A23187, nor 4 beta-phorbol dibutyrate could substitute for angiotensin. The effects of isoproterenol were greatest with submaximal concentrations of angiotensin, whereas maximal concentrations of angiotensin were affected little. Isoproterenol did not substantially increase the formation of [3H]inositol triphosphate or the ratio of isomers [3H]inositol 1,4,5-trisphosphate and [3H]inositol 1,3,4-trisphosphate formed in response to angiotensin. Isoproterenol also enhanced the phase of Ca2+ mobilization involving Ca2+ entry which is consistent with the previously proposed functional linkage between receptor-regulated Ca2+ release and Ca2+ entry. These findings suggest that isoproterenol may act by increasing the sensitivity of the endoplasmic reticulum to the Ca2+-releasing action of inositol 1,4,5-trisphosphate.

Angiotensin II↗