Extracellular ATP increases cytosolic calcium in cultured rat renal arterial smooth muscle cells

IKCa channels control breast cancer metabolism including AMPK-driven autophagy

Ca²⁺-activated K⁺ channels of intermediate conductance (IK) are frequently overexpressed in breast cancer (BC) cells, while IK channel depletion reduces BC cell proliferation and tumorigenesis. This raises the question, of whether and mechanistically how IK activity interferes with the metabolic activity and energy consumption rates, which are fundamental for rapidly growing cells. Using BC cells obtained from MMTV-PyMT tumor-bearing mice, we show that both, glycolysis and mitochondrial ATP-production are reduced in cells derived from IK-deficient breast tumors. Loss of IK altered the sub-/cellular K⁺- and Ca²⁺- homeostasis and mitochondrial membrane potential, ultimately resulting in reduced ATP-production and metabolic activity. Consequently, we find that BC cells lacking IK upregulate AMP-activated protein kinase activity to induce autophagy compensating the glycolytic and mitochondrial energy shortage. Our results emphasize that IK by modulating cellular Ca²⁺- and K⁺-dynamics contributes to the remodeling of metabolic pathways in cancer. Thus, targeting IK channel might disturb the metabolic activity of BC cells and reduce malignancy.

The effect of extracellular ATP on rat uterine contraction from different gestational stages and its possible mechanisms of action

BACKGROUND

The mechanisms underlying the onset of labor are not fully understood. Extracellular adenosine 5'-triphosphate (ATP) is known to cause uterine contractions in different species but the exact underlying mechanisms are poorly investigated to date. The aims of this study were to investigate the effect of extracellular ATP on spontaneous uterine contractions from different gestational stages and to elucidate its possible underlying mechanisms.

METHODS

Longitudinal uterine strips were obtained from rats in different gestational stages (nonpregnant, late-pregnant, and term-pregnant). The effects of 1 mM ATP were examined on uterine contractions generated spontaneously, depolarized by high-KCl (60 mM), induced by oxytocin (5 nM), in the presence of high external Ca2+, or in the absence of external Ca2+.

RESULTS

Application of 1 mM extracellular ATP significantly increased the force of spontaneous contraction in uterine strips obtained from all gestational stages with prominent increase in term-pregnant rats compared to other gestations. ATP significantly increased the force induced by depolarization (122%, p=0.010, n=6), oxytocin (129%, p=0.001, n=7), high-Ca2+ (145%, p=0.005, n=6) and it was able to cause transient contraction in the absence of external Ca2+ (33%, p<0.01).

CONCLUSIONS

Extracellular ATP is able to increase the force and frequency of uterine contractions and its effect increases with the progression of pregnancy and it involves Ca2+ influx and release. These findings open a new window for clinicians to consider ATP as a therapeutic target to control the uterine activity during difficult labors.

Changes in expression of P2X7 receptors in rat myometrium at different gestational stages and the mechanism of ATP-induced uterine contraction

AIMS

Given the importance of ATP in the control of uterine activity for successful labor and involution, this study was performed to measure the level of P2X7 receptors (P2X7Rs) in rat myometrium at different gestational stages and to investigate the mechanisms of ATP-induced uterine contraction.

MATERIALS AND METHODS

Myometrial tissues were obtained from rats at different gestational stages and the level of P2X7Rs was measured by ELISA. In other experiments, the effect of 1 mM ATP was tested on spontaneous contraction and the underlying mechanisms were investigated.

KEY FINDINGS

P2X7Rs were expressed in nonpregnant uterine tissues, progressively increased throughout pregnancy, and markedly peaked during postpartum involution. ATP significantly increased the force of spontaneous contraction in all uterine strips from different gestational stages with marked increase during labor and postpartum. ATP could not maintain the force when external Ca²⁺ was removed. In addition, ATP was able to cause tonic transient contraction in the absence of external Ca²⁺.

SIGNIFICANCE

P2X7Rs are functionally regulated and contributed to ATP-induced uterine contraction. The sensitivity of the myometrium to ATP increases as pregnancy progresses and it involves Ca²⁺ influx and Ca²⁺ release pathways. The clear effects of ATP on contractility suggest its physiological requirement for successful labor and postpartum involution.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

ATP, P2 receptors and the renal microcirculation

Purinoceptors are rapidly becoming recognised as important regulators of tissue and organ function. Renal expression of P2 receptors is broad and diverse, as reflected by the fact that P2 receptors have been identified in virtually every major tubular/vascular element. While P2 receptor expression by these renal structures is recognised, the physiological functions that they serve remains to be clarified. Renal vascular P2 receptor expression is complex and poorly understood. Evidence suggests that different complements of P2 receptors are expressed by individual renal vascular segments. This unique distribution has given rise to the postulate that P2 receptors are important for renal vascular function, including regulation of preglomerular resistance and autoregulatory behaviour. More recent studies have also uncovered evidence that hypertension reduces renal vascular reactivity to P2 receptor stimulation in concert with compromised autoregulatory capability. This review will consolidate findings related to the role of P2 receptors in regulating renal microvascular function and will present areas of controversy related to the respective roles of ATP and adenosine in autoregulatory resistance adjustments.

Succinate receptor GPR91 provides a direct link between high glucose levels and renin release in murine and rabbit kidney

Diabetes mellitus is the most common and rapidly growing cause of end-stage renal disease in developed countries. A classic hallmark of early diabetes mellitus includes activation of the renin-angiotensin system (RAS), which may lead to hypertension and renal tissue injury, but the mechanism of RAS activation is elusive. Here we identified a paracrine signaling pathway in the kidney in which high levels of glucose directly triggered the release of the prohypertensive hormone renin. The signaling cascade involved the local accumulation of succinate and activation of the kidney-specific G protein-coupled metabolic receptor, GPR91, in the glomerular endothelium as observed in rat, mouse, and rabbit kidney sections. Elements of signal transduction included endothelial Ca2+, the production of NO and prostaglandin (PGE2), and their paracrine actions on adjacent renin-producing cells. This GPR91 signaling cascade may serve to modulate kidney function and help remove metabolic waste products through renal hyperfiltration, and it could also link metabolic diseases, such as diabetes, or metabolic syndrome with RAS overactivation, systemic hypertension, and organ injury.

P2 receptor regulation of [Ca2+]i in cultured mouse mesangial cells

Experiments were performed to establish the pharmacological profile of purinoceptors and to identify the signal transduction pathways responsible for increases in intracellular calcium concentration ([Ca(2+)](i)) for cultured mouse mesangial cells. Mouse mesangial cells were loaded with fura 2 and examined using fluorescent spectrophotometry. Basal [Ca(2+)](i) averaged 102 +/- 2 nM (n = 346). One hundred micromolar concentrations of ATP, ADP, 2',3'-(benzoyl-4-benzoyl)-ATP (BzATP), ATP-gamma-S, and UTP in normal Ca(2+) medium evoked peak increases in [Ca(2+)](i) of 866 +/- 111, 236 +/- 18, 316 +/- 26, 427 +/- 37, and 808 +/- 73 nM, respectively. UDP or 2-methylthio-ATP (2MeSATP) failed to elicit significant increases in [Ca(2+)](i), whereas identical concentrations of adenosine, AMP, and alpha,beta-methylene ATP (alpha,beta-MeATP) had no detectable effect on [Ca(2+)](i). Removal of Ca(2+) from the extracellular medium had no significant effect on the peak increase in [Ca(2+)](i) induced by ATP, ADP, BzATP, ATP-gamma-S, or UTP compared with normal Ca(2+); however, Ca(2+)-free conditions did accelerate the rate of decline in [Ca(2+)](i) in cells treated with ATP and UTP. [Ca(2+)](i) was unaffected by membrane depolarization with 143 mM KCl. Western blot analysis for P2 receptors revealed expression of P2X(2), P2X(4), P2X(7), P2Y(2), and P2Y(4) receptors. No evidence of P2X(1) and P2X(3) receptor expression was detected, whereas RT-PCR analysis reveals mRNA expression for P2X(1), P2X(2), P2X(3), P2X(4), P2X(7), P2Y(2), and P2Y(4) receptors. These data indicate that receptor-specific P2 receptor activation increases [Ca(2+)](i) by stimulating calcium influx from the extracellular medium and through mobilization of Ca(2+) from intracellular stores in cultured mouse mesangial cells.

Mechanism of eNOS gene transfer inhibition of vascular smooth muscle cell proliferation

Endothelial nitric oxide synthase (eNOS) is responsible for the production of nitric oxide (NO) in blood vessels. NO has been shown to be involved in the inhibition of vascular smooth muscle cell (VSMC) proliferation. In the present study, the eNOS gene was transferred into rat aortic smooth muscle cells by using an adenoviral vector, and the effect of endogenously produced NO on VSMC proliferation was investigated. The presence of eNOS in eNOS-transfected cells was confirmed by immunocytochemistry and Western blot analysis. eNOS transfection resulted in inhibition of VSMC proliferation. This effect was accompanied by increased levels of p53 and p21. This effect was abrogated in the presence of the protein kinase A (PKA) inhibitor Rp-8-bromoadenosine 3',5'-cyclic monophosphothioate. The increased levels of p53 and p21 observed in eNOS-transfected cells were reduced in the presence of the PKA inhibitor. These data suggest that p21 and p53 play a role in the inhibition of proliferation in eNOS-transfected cells and that levels of these two proteins are regulated by PKA.

Phospholipase D contributes to transmural pressure control of prorenin processing in juxtaglomerular cell

This study was designed to delineate the involvement of phospholipase C (PLC) and phospholipase D (PLD) in transmural pressure control of renin synthesis and secretion. Primary cultures of rat juxtaglomerular (JG) cells were applied to a transmural pressure-loading apparatus for 12 hours, and the renin secretion rate (RSR), active renin content (ARC), and total (active + inactive) renin content (TRC) were determined. Under control conditions (n=5), transmural pressure decreased RSR (78.1 +/- 3.0 and 64.6 +/- 4.4% for 0 or 40 mm Hg, respectively; P<0.05) and ARC (42.8 +/- 3.3 and 26.0 +/- 3.9 ng of angiotensin I per hour per million cells for 0 or 40 mm Hg, respectively; P<0.05) but did not have a significant effect on TRC (99.5 +/- 6.7 and 89.2 +/- 4.6 ng of angiotensin I per hour per million cells for 0 or 40 mm Hg, respectively). Treatment with PLC inhibitors, 2-nitro-4-carboxyphenyl-N,N-diphenyl-carbamate (200 micromol/L) and U73122 (10 micromol/L) did not alter RSR but prevented the RSR decrease with transmural pressure, whereas neither 2-nitro-4-carboxyphenyl-N,N-diphenyl-carbamate nor U73122 altered ARC, TRC, or the decrease in ARC with transmural pressure. Experiments were also performed using JG cells (n=5) treated with a PLD inhibitor, 4-(2-aminoethyl)-benzensulfonyl fluoride (AEBSF, 100 micromol/L). Treatment with AEBSF did not influence basal levels of RSR, ARC, and TRC or the RSR decrease with transmural pressure. However, AEBSF did inhibit the decrease in ARC with transmural pressure. These results indicate that transmural pressure inhibits renin secretion via PLC-dependent pathways and prevents conversion of inactive renin to active renin via PLD-dependent mechanisms in JG cells.

P2 receptors in regulation of renal microvascular function

In the last 10-15 years, interest in the physiological role of P2 receptors has grown rapidly. Cellular, tissue, and organ responses to P2 receptor activation have been described in numerous in vivo and in vitro models. The purpose of this review is to provide an update of the recent advances made in determining the involvement of P2 receptors in the control of renal hemodynamics and the renal microcirculation. Special attention will be paid to work published in the last 5-6 years directed at understanding the role of P2 receptors in the physiological control of renal microvascular function. Several investigators have begun to evaluate the effects of P2 receptor activation on renal microvascular function across several species. In vivo and in vitro evidence consistently supports the hypothesis that P2 receptor activation by locally released extracellular nucleotides influences microvascular function. Extracellular nucleotides selectively influence preglomerular resistance without having an effect on postglomerular tone. P2 receptor inactivation blocks autoregulatory behavior whereas responsiveness to other vasoconstrictor agonists is retained. P2 receptor stimulation activates multiple intracellular signal transduction pathways in preglomerular smooth muscle cells and mesangial cells. Renal microvascular cells and mesangial cells express multiple subtypes of P2 receptors; however, the specific role each plays in regulating vascular and mesangial cell function remains unclear. Accordingly, the results of studies performed to date provide strong support for the hypothesis that P2 receptors are important contributors to the physiological regulation of renal microvascular and/or glomerular function.

Renal microvascular effects of P2 receptor stimulation

1. The field of extracellular nucleotides and purinoceptors has undergone a resurgence of interest and enthusiasm in the past decade. More and more investigators are probing the physiological and pathophysiological roles of P2 receptors in virtually every organ system, including the kidney. 2. With this renewed interest has come a new appreciation for the roles extracellular adenine nucleotides can play in regulating or modulating renal function. In the past 5 years, investigators have provided compelling evidence that extracellular nucleotides, working through activation of P2 purinoceptors, have a significant impact on renal microvascular function, mesangial cell function and on renal epithelial transport. 3. Evidence has been uncovered that implicates P2 receptor activation in mediating renal microvascular autoregulatory behaviour. Locally released ATP has a direct paracrine and/or autocrine effect modulating renal epithelial transporters and tubular epithelial channels to influence tubular fluid composition. 4. While the specific roles of extracellular nucleotides and their receptors in the kidney have not been absolutely identified, it now appears clear that endogenously released ATP may play a significant role in regulating kidney function. 5. The purpose of the present review is to update our current understanding of the effect of P2 receptor activation on renal microvascular function and to detail the signal transduction mechanisms known to be involved.

Integrin mobilizes intracellular Ca(2+) in renal vascular smooth muscle cells

Peptides with the Arg-Gly-Asp (RGD) motif induce vasoconstriction in rat afferent arterioles by increasing the intracellular Ca(2+) concentration ([Ca(2+)](i)) in vascular smooth muscle cells (VSMC). This finding suggests that occupancy of integrins on the plasma membrane of VSMC might affect vascular tone. The purpose of this study was to determine whether occupancy of integrins by exogenous RGD peptides initiates intracellular Ca(2+) signaling in cultured renal VSMC. When smooth muscle cells were exposed to 0.1 mM hexapeptide GRGDSP, [Ca(2+)](i) rapidly increased from 91 +/- 4 to 287 +/- 37 nM and then returned to the baseline within 20 s (P < 0.05, 34 cells/5 coverslips). In controls, the hexapeptide GRGESP did not trigger Ca(2+) mobilization. Local application of the GRGDSP induced a regional increase of cytoplasmic [Ca(2+)](i), which propagated as Ca(2+) waves traveling across the cell and induced a rapid elevation of nuclear [Ca(2+)](i). Spontaneous recurrence of smaller-amplitude Ca(2+) waves were found in 20% of cells examined after the initial response to RGD-containing peptides. Blocking dihydropyridine-sensitive Ca(2+) channels with nifedipine or removal of extracellular Ca(2+) did not inhibit the RGD-induced Ca(2+) mobilization. However, pretreatment of 20 microM ryanodine completely eliminated the RGD-induced Ca(2+) mobilization. Anti-beta(1) and anti-beta(3)-integrin antibodies with functional blocking capability simulate the effects of GRGDSP in [Ca(2+)](i). Incubation with anti-beta(1)- or beta(3)-integrin antibodies inhibited the increase in [Ca(2+)](i) induced by GRGDSP. We conclude that exogenous RGD-containing peptides induce release of Ca(2+) from ryanodine-sensitive Ca(2+) stores in renal VSMC via integrins, which can trigger cytoplasmic Ca(2+) waves propagating throughout the cell.

Iloprost inhibits inositol-1,4,5-trisphosphate-mediated calcium mobilization stimulated by angiotensin II in cultured preglomerular vascular smooth muscle cells

In a previous study of cultured preglomerular vascular smooth muscle cells, it was demonstrated that, although the stable prostacyclin analog iloprost alone had no effect on the intracellular calcium concentration ([Ca2+](i)), it did significantly attenuate the increase in [Ca2+](i) stimulated by angiotensin II (AngII). In this study, the mechanisms by which iloprost interacts with calcium signaling pathways stimulated by AngII were examined. [Ca2+](i) was assessed using the calcium-sensitive fluorescent dye fura-2. Initial studies identified two major components of the [Ca2+](i) response to AngII in this homogeneous preparation of vascular smooth muscle cells from renal resistance vessels. Mobilization of internal stores was evident as an immediate TMB-8-sensitive peak increase in [Ca2+](i) (52 +/- 6 to 297 +/- 26 nM, P: < 0.001) in a calcium-free medium. After [Ca2+](i) had returned to baseline levels during continued AngII stimulation, a nifedipine-sensitive entry pathway was revealed by the sustained stimulatory effect of added external calcium, which increased [Ca2+](i) to 112 +/- 13 nM (P: < 0.001). Coadministration of iloprost with AngII attenuated both the immediate peak (154 +/- 14 nM) and sustained plateau (61 +/- 9 nM) phases. Increases in endogenous levels of cAMP induced by the phosphodiesterase inhibitor milrinone mirrored the actions of iloprost, suggesting that the prostacyclin analog exerted its actions via cAMP activation. Blockade of cAMP-dependent protein kinase with KT 5720 reversed the effects of both iloprost and milrinone. When iloprost or milrinone was introduced after the initial mobilization peak had dissipated, the plateau phase of calcium entry was unchanged (92 +/- 9 nM). The concept that iloprost does not directly modulate calcium entry was further supported by data showing that the activation of L-type calcium channels by BAY-K 8644 was unchanged during iloprost treatment. On the basis of the observation that iloprost did not alter thapsigargin stimulation of Ca(2+)-ATPase activity, it is concluded that the actions of cAMP are distinct from increasing calcium uptake into the sarcoplasmic reticulum. This study provides new information on the ability of iloprost to primarily attenuate inositol-1,4,5-triphosphate-mediated calcium mobilization via cAMP, with secondary inhibition of L-type calcium entry channels. These data clarify the mechanism by which prostaglandins buffer AngII constriction in resistance arterioles.

Vasodilator responses to ATP and UTP are not dependent on nitric oxide release, K+ATP channel activation, or the release of vasodilator prostaglandins in the hindlimb vascular bed of the cat

The effects of the purinergic agonists, ATP, ATPγS, UTP, and 2-Met-Thio AP, were investigated in the hindlimb vascular bed of the cat. Under constant-flow conditions, injections of the purinergic agonists into the perfusion circuit elicited dose-related decreases in perfusion pressure. The order of potency was 2-Met-Thio ATP > ATPγS > ATP > UTP. In contrast, injections of GTPγS, cAMP, UDP, and UMP had no effect. Vasodilator responses to ATP, ATPγS, UTP, and 2-Met-Thio ATP were increased in duration by the cAMP phosphodiesterase inhibitor rolipram, whereas the cGMP phosphodiesterase inhibitor zaprinast had no effect. Responses to the purinergic agonists were not altered by nitric oxide synthase inhibitors, K+ATP channel antagonists, cyclooxygenase inhibitors, or agents that interfere with the actions of the adrenergic nervous system. These data suggest that ATP, ATPγS, UTP, and 2-Met-Thio ATP dilate the hindlimb vascular bed by a direct cAMP-dependent mechanism, and that the release of nitric oxide, vasodilator prostaglandins, K+ATP channel opening, or an inhibitory effect on the adrenergic nervous system play little, if any, role in mediating or modulating responses to the purinergic agonists in the hindlimb circulation of the cat.Key words: purinergic agonists, P2 purinergic receptors, cAMP-dependent vasodilator activity, adrenergic system, nitric oxide prostaglandins.

Effect of nitric oxide on calcium-induced calcium release in coronary arterial smooth muscle

The present study was designed to determine whether nitric oxide (NO)-induced reduction of [Ca(2+)](i) is associated with Ca(2+)-induced Ca(2+) release (CICR) in coronary arterial smooth muscle cells (CASMCs). Caffeine was used as a CICR activator to induce Ca(2+) release in these cells. The effects of NO donor, sodium nitroprusside (SNP), on caffeine-induced Ca(2+) release were examined in freshly dissociated bovine CASMCs using single cell fluorescence microscopic spectrometry. The effects of NO donor on caffeine-induced coronary vasoconstriction were examined by isometric tension recordings. Caffeine, a CICR or ryanodine receptor (RYR) activator, produced a rapid Ca(2+) release with a 330 nM increase in [Ca(2+)](i). Pretreatment of the CASMCs with SNP, CICR inhibitor tetracaine or RYR blocker ryanodine markedly decreased caffeine-induced Ca(2+) release. Addition of caffeine to the Ca(2+)-free bath solution produced a transient coronary vasoconstriction. SNP, tetracaine and ryanodine, but not guanylyl cyclase inhibitor, ODQ, significantly attenuated caffeine-induced vasoconstriction. These results suggest that CICR is functioning in CASMCs and participates in the vasoconstriction in response to caffeine-induced Ca(2+) release and that inhibition of CICR is of importance in mediating the vasodilator response of coronary arteries to NO.

ATP-mediated Ca2+ signaling in preglomerular smooth muscle cells

We performed studies to determine the effect of extracellular ATP on the intracellular Ca2+ concentration ([Ca2+]i) in freshly isolated microvascular smooth muscle cells (MVSMC). Suspensions of preglomerular MVSMC were prepared by enzymatic digestion and loaded with fura 2. Single cells were studied using a microscope-based fluorescence spectrophotometer during superfusion of a physiological salt solution with 1.8 mM Ca2+ and during exposure to similar solutions containing ATP. Under control conditions, baseline [Ca2+]i averaged 107 +/- 6 nM (n = 86 cells from 34 animals). ATP administration elicited concentration-dependent increases in [Ca2+]i. Exposure to ATP concentrations of 1, 10, and 100 microM increased intracellular Ca2+ to peak concentrations of 133 +/- 20, 338 +/- 37, and 367 +/- 35 nM, respectively (P < 0.05 vs. respective baseline). Steady-state [Ca2+]i increased to 113 +/- 15, 150 +/- 16 (P < 0.05 vs. baseline), and 180 +/- 12 nM (P < 0.05 vs. baseline) for the same groups. The [Ca2+]i response to ATP was also assessed in the absence of extracellular Ca2+ and during blockade of L-type Ca2+ channels with diltiazem. In these studies, exposure to 100 microM ATP induced a transient peak increase in [Ca2+]i with the plateau phase being totally abolished under Ca2+-free conditions and markedly attenuated during Ca2+ channel blockade, respectively. These data indicate that ATP-mediated P2-receptor activation increases [Ca2+]i in freshly isolated preglomerular MVSMC by stimulating Ca2+ release from intracellular stores, in addition to stimulating the influx of extracellular Ca2+ through voltage-gated L-type Ca2+ channels.

Purinoceptor-mediated calcium signaling in preglomerular smooth muscle cells

-The current studies were performed to determine the contribution of calcium mobilization and voltage-dependent calcium influx to the increase in [Ca2+]i elicited by ATP and UTP. Suspensions of freshly isolated smooth muscle cells were prepared from preglomerular microvessels by enzymatic digestion and loaded with the Ca2+-sensitive dye fura 2. The effect of ATP and UTP on [Ca2+]i was studied on single cells with standard microscope-based fluorescence photometry techniques. Resting [Ca2+]i averaged 80+/-3 nmol/L (n=219 single cells from 58 dispersions). ATP (100 micromol/L) increased [Ca2+]i to a peak value of 845+/-55 nmol/L (n=70 single cells from 38 dispersions) before stabilizing at 124+/-81 nmol/L. Similarly, 100 micromol/L UTP (n=39 single cells from 26 dispersions) stimulated a peak increase in [Ca2+]i of 1426+/-584 nmol/L before reaching a stable plateau of 123+/-10 nmol/L. The [Ca2+]i response to ATP and UTP was also assessed in the absence of extracellular calcium. In these studies, exposure to 100 micromol/L ATP induced a transient peak increase in [Ca2+]i, with the plateau phase being totally abolished. In contrast, exposure to 100 micromol/L UTP under calcium-free conditions resulted in no detectable change in the UTP-mediated increase in [Ca2+]i. The role of L-type calcium channels in the response was assessed with the calcium channel antagonist diltiazem. Incubation with diltiazem (10 micromol/L) markedly reduced the response to ATP, whereas the response to UTP was only slightly reduced. These data demonstrate that both ATP and UTP directly stimulate a biphasic increase in [Ca2+]i in renal microvascular smooth muscle cells. Furthermore, the data suggest that the elevation of [Ca2+]i elicited by ATP is largely dependent on calcium influx through L-type calcium channels, whereas the response to UTP appears to derive primarily from mobilization of calcium from intracellular stores.

Direct assessment of renal microvascular responses to P2-purinoceptor agonists

Studies were performed to determine the responsiveness of rat juxtamedullary afferent arterioles to receptor-selective P2-purinoceptor agonists. Experiments were performed in vitro using the blood perfused juxtamedullary nephron technique, combined with videomicroscopy. Renal perfusion pressure was set at 110 mmHg and held constant. Basal afferent arteriolar diameter averaged 22.0 +/- 0.6 microns (n = 69). Stimulation with 0.1, 1.0, 10, and 100 microM ATP (n = 10) elicited a concentration-dependent vasoconstriction averaging 8 +/- 2, 17 +/- 2, 21 +/- 4, and 23 +/- 5%, respectively. A nearly identical afferent arteriolar vasoconstriction was observed in response to the P2X-selective agonist beta,gamma-methylene ATP (n = 10); however, another P2X agonist, alpha,beta-methylene ATP, evoked marked receptor desensitization (n = 10). Vessel diameter decreased by approximately 7 +/- 2, 16 +/- 2, 23 +/- 3, and 22 +/- 3%, respectively, over the same concentration range. The P2Y-selective agonist, 2-methylthio-ATP, evoked only a modest vasoconstriction, whereas UTP and adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S) reduced afferent diameter markedly at concentrations > 1.0 microM. Afferent arteriolar diameter decreased by 5 +/- 4, 31 +/- 8, and 72 +/- 8% during UTP administration (n = 7) at concentrations of 1.0, 10, and 100 microM, respectively. Similarly, ATP gamma S (n = 6) decreased afferent diameter by 16 +/- 2, 58 +/- 8, and 98 +/- 3%, respectively, over the same concentration range. Nitric oxide synthesis inhibition with N omega-nitro-L-arginine did not significantly alter the afferent arteriolar response to ATP but did potentiate ATP-mediated arcuate artery vasoconstriction. The following data suggest the presence of multiple P2 receptors on juxtamedullary afferent arterioles and are consistent with classification of those receptors as members of the P2X- and P2Y2 (P2U)-receptor subtypes.

Purinoceptor-mediated regulation of the renal microvasculature

1. Recent evidence supports the contention that numerous paracrine factors are released into the intrarenal interstitial environment where they can influence renal microvascular and tubular function. Studies have shown that P1 and P2 purinoceptor activation by adenosine and ATP, respectively, can selectively evoke alterations in renal microvascular diameter. 2. Vasoconstrictor responses to extracellular ATP, which are almost exclusively preglomerular, are biphasic, develop rapidly and are rapidly reversible. ATP-mediated afferent arteriolar vasoconstriction is largely dependent on the influx of extracellular calcium and the sustained vasoconstriction is maintained by calcium influx through voltage-dependent L-type calcium channels. 3. Microvascular responses to adenosine include vasoconstriction at low concentrations and vasodilation at higher concentrations. Adenosine elicits a vasoconstrictor response which develops more slowly than responses to ATP and the vasoconstriction is elicited from both the preglomerular and postglomerular arterioles. 4. Studies were performed to evaluate the possible involvement of P2 purinoceptor activation in the renal vascular autoregulatory response. Inactivation of P2 receptor-dependent preglomerular microvascular responses by purinoceptor desensitization or purinoceptor saturation completely abolished autoregulatory adjustments in afferent arteriolar calibre. 5. These studies demonstrate that the renal microvasculature is responsive to purinoceptor activation. Furthermore, the data support an important role for the involvement of intrarenal P2 receptors in the regulation of renal haemodynamics.