Ca2+ channels, ryanodine receptors and Ca(2+)-activated K+ channels: a functional unit for regulating arterial tone

Local calcium transients ('Ca2+ sparks') are thought to be elementary Ca2+ signals in heart, skeletal and smooth muscle cells. Ca2+ sparks result from the opening of a single, or the coordinated opening of many, tightly clustered ryanodine receptor (RyR) channels in the sarcoplasmic reticulum (SR). In arterial smooth muscle, Ca2+ sparks appear to be involved in opposing the tonic contraction of the blood vessel. Intravascular pressure causes a graded membrane potential depolarization to approximately -40 mV, an elevation of arterial wall [Ca2+]i and contraction ('myogenic tone') of arteries. Ca2+ sparks activate calcium-sensitive K+ (KCa) channels in the sarcolemmal membrane to cause membrane hyperpolarization, which opposes the pressure induced depolarization. Thus, inhibition of Ca2+ sparks by ryanodine, or of KCa channels by iberiotoxin, leads to membrane depolarization, activation of L-type voltage-gated Ca2+ channels, and vasoconstriction. Conversely, activation of Ca2+ sparks can lead to vasodilation through activation of KCa channels. Our recent work is aimed at studying the properties and roles of Ca2+ sparks in the regulation of arterial smooth muscle function. The modulation of Ca2+ spark frequency and amplitude by membrane potential, cyclic nucleotides and protein kinase C will be explored. The role of local Ca2+ entry through voltage-dependent Ca2+ channels in the regulation of Ca2+ spark properties will also be examined. Finally, using functional evidence from cardiac myocytes, and histological evidence from smooth muscle, we shall explore whether Ca2+ channels, RyR channels, and KCa channels function as a coupled unit, through Ca2+ and voltage, to regulate arterial smooth muscle membrane potential and vascular tone.

Laparoscopic intraperitoneal onlay inguinal herniorrhaphy

BACKGROUND

This study presents intermediate follow-up data on a randomized prospective series of patients undergoing either a modified laparoscopic intraperitoneal onlay mesh herniorrhaphy (IPOM) or conventional anterior inguinal herniorrhaphy (CH).

METHODS

All patients from two university affiliated hospitals with primary or recurrent inguinal hernias were recruited for randomization to either the IPOM technique utilizing a meshed expanded polytetrafluorethylene (ePTFE) soft tissue patch or CH. Follow-up data were gathered from postoperative clinic visits and telephone and mail surveys.

RESULTS

Previously reported early recurrence and complication rates at a mean follow-up of 8 months were 1 of 30 (3%) and 5 of 30 (17%) for IPOM, and 2 of 28 (7%) and 5 of 28 (18%) for CH. Intermediate follow-up with 50 (23 IPOM and 27 CH) of the original 58 patients (86%) at a mean of 41 months reveals a recurrence rate of 10 of 23 (43%) for the IPOM group and 4 of 27 (15%) for the CH group (P = 0.053). Five delayed complications occurred in 4 IPOM patients (port site hernia 4, painful neuroma 1), while 2 delayed complications (unilateral testicular atrophy 2) occurred in 2 patients in the CH group. One IPOM versus 5 CH patients subsequently developed previously unrecognized contralateral hernias. There was 1 death unrelated to previous herniorrhaphy in each group.

CONCLUSIONS

IPOM recurrence rates (43%) at a mean follow-up of 41 months are excessively high when compared with CH (15%) or with preliminary results of IPOM at 8 months of follow-up (3%). Despite reduced perioperative pain and disability and promising preliminary results in the IPOM group, these intermediate follow-up data strongly suggest that the IPOM technique should not be used for repair of inguinal hernias.

Ontogeny of local sarcoplasmic reticulum Ca2+ signals in cerebral arteries: Ca2+ sparks as elementary physiological events

Ca2+ release through ryanodine receptors (RyRs) in the sarcoplasmic reticulum is a key element of excitation-contraction coupling in muscle. In arterial smooth muscle, Ca2+ release through RyRs activates Ca2+-sensitive K+ (KCa) channels to oppose vasoconstriction. Local Ca2+ transients ("Ca2+ sparks"), apparently caused by opening of clustered RyRs, have been observed in smooth and striated muscle. We explored the fundamental issue of whether RyRs generate Ca2+ sparks to regulate arterial smooth muscle tone by examining the function of RyRs during ontogeny of arteries in the brain. In the present study, Ca2+ sparks were measured using the fluorescent Ca2+ indicator fluo-3 combined with laser scanning confocal microscopy. Diameter and arterial wall [Ca2+] measurements obtained from isolated pressurized arteries were also used in this study to provide functional insights. Neonatal arteries (<1 day postnatal), although still proliferative, have the molecular components for excitation-contraction coupling, including functional voltage-dependent Ca2+ channels, RyRs, and KCa channels and also constrict to elevations in intravascular pressure. Despite having functional RyRs, Ca2+ spark frequency in intact neonatal arteries was approximately 1/100 of adult arteries. In marked contrast to adult arteries, neonatal arteries did not respond to inhibitors of RyRs and KCa channels. These results support the hypothesis that RyRs organize during postnatal development to cause Ca2+ sparks, and RyRs must generate Ca2+ sparks to regulate the function of the intact tissue.

Voltage dependence of Ca2+ sparks in intact cerebral arteries

Ca2+ sparks have been previously described in isolated smooth muscle cells. Here we present the first measurements of local Ca2+ transients ("Ca2+ sparks") in an intact smooth muscle preparation. Ca2+ sparks appear to result from the opening of ryanodine-sensitive Ca2+ release (RyR) channels in the sarcoplasmic reticulum (SR). Intracellular Ca2+ concentration ([Ca2+]i) was measured in intact cerebral arteries (40-150 micron in diameter) from rats, using the fluorescent Ca2+ indicator fluo 3 and a laser scanning confocal microscope. Membrane potential depolarization by elevation of external K+ from 6 to 30 mM increased Ca2+ spark frequency (4. 3-fold) and amplitude (approximately 2-fold) as well as global arterial wall [Ca2+]i (approximately 1.7-fold). The half time of decay ( approximately 50 ms) was not affected by membrane potential depolarization. Ryanodine (10 microM), which inhibits RyR channels and Ca2+ sparks in isolated cells, and thapsigargin (100 nM), which indirectly inhibits RyR channels by blocking the SR Ca2+-ATPase, completely inhibited Ca2+ sparks in intact cerebral arteries. Diltiazem, an inhibitor of voltage-dependent Ca2+ channels, lowered global [Ca2+]i and Ca2+ spark frequency and amplitude in intact cerebral arteries in a concentration-dependent manner. The frequency of Ca2+ sparks (<1 s-1 . cell-1), even under conditions of steady depolarization, was too low to contribute significant amounts of Ca2+ to global Ca2+ in intact arteries. These results provide direct evidence that Ca2+ sparks exist in quiescent smooth muscle cells in intact arteries and that changes of membrane potential that would simulate physiological changes modulate both Ca2+ spark frequency and amplitude in arterial smooth muscle.

Frequency modulation of Ca2+ sparks is involved in regulation of arterial diameter by cyclic nucleotides

Forskolin, which elevates cAMP levels, and sodium nitroprusside (SNP) and nicorandil, which elevate cGMP levels, increased, by two- to threefold, the frequency of subcellular Ca2+ release ("Ca2+ sparks") through ryanodine-sensitive Ca2+ release (RyR) channels in the sarcoplasmic reticulum (SR) of myocytes isolated from cerebral and coronary arteries of rats. Forskolin, SNP, nicorandil, dibutyryl-cAMP, and adenosine increased the frequency of Ca(2+)-sensitive K+ (KCa) currents ["spontaneous transient outward currents" (STOCs)] by two- to threefold, consistent with Ca2+ sparks activating STOCs. These agents also increased the mean amplitude of STOCs by 1.3-fold, an effect that could be explained by activation of KCa channels, independent of effects on Ca2+ sparks. To test the hypothesis that cAMP could act to dilate arteries through activation of the Ca2+ spark-->KCa channel pathway, the effects of blockers of KCa channels (iberiotoxin) and of Ca2+ sparks (ryanodine) on forskolin-induced dilations of pressurized cerebral arteries were examined. Forskolin-induced dilations were partially inhibited by iberiotoxin and ryanodine (with no additive effects) and were entirely prevented by elevating external K+. Forskolin lowered average Ca2+ in pressurized arteries while increasing ryanodine-sensitive, caffeine-induced Ca2+ transients. These experiments suggest a new mechanism for cyclic nucleotide-mediated dilations through an increase in Ca2+ spark frequency, caused by effects on SR Ca2+ load and possibly on the RyR channel, which leads to increased STOC frequency, membrane potential hyperpolarization, closure of voltage-dependent Ca2+ channels, decrease in arterial wall Ca2+, and, ultimately, vasodilation.

Ryanodine receptors regulate arterial diameter and wall [Ca2+] in cerebral arteries of rat via Ca2+-dependent K+ channels

1. The effects of inhibitors of ryanodine-sensitive calcium release (RyR) channels in the sarcoplasmic reticulum (SR) and Ca2+-dependent potassium (KCa) channels on the membrane potential, intracellular [Ca2+], and diameters of small pressurized (60 mmHg) cerebral arteries (100-200 micron) were studied using digital fluorescence video imaging of arterial diameter and wall [Ca2+], combined with microelectrode measurements of arterial membrane potential. 2. Ryanodine (10 microM), an inhibitor of RyR channels, depolarized by 9 mV, increased intracellular [Ca2+] by 46 nM and constricted pressurized (to 60 mmHg) arteries with myogenic tone by 44 micron (approximately 22 %). Iberiotoxin (100 nM), a blocker of KCa channels, under the same conditions, depolarized the arteries by 10 mV, increased arterial wall calcium by 51 nM, and constricted by 37 micron (approximately 19 %). The effects of ryanodine and iberiotoxin were not additive and were blocked by inhibitors of voltage-dependent Ca2+ channels. 3. Caffeine (10 mM), an activator of RyR channels, transiently increased arterial wall [Ca2+] by 136 +/- 9 nM in control arteries and by 158 +/- 12 nM in the presence of iberiotoxin. Caffeine was relatively ineffective in the presence of ryanodine, increasing [calcium] by 18 +/- 5 nM. 4. In the presence of blockers of voltage-dependent Ca2+ channels (nimodipine, diltiazem), ryanodine and inhibitors of the SR calcium ATPase (thapsigargin, cyclopiazonic acid) were without effect on arterial wall [Ca2+] and diameter. 5. These results suggest that local Ca2+ release originating from RyR channels (Ca2+ sparks) in the SR of arterial smooth muscle regulates myogenic tone in cerebral arteries solely through activation of KCa channels, which regulate membrane potential through tonic hyperpolarization, thus limiting Ca2+ entry through L-type voltage-dependent Ca2+ channels. KCa channels therefore act as a negative feedback control element regulating arterial diameter through a reduction in global intracellular free [Ca2+].

Regulation of arterial diameter and wall [Ca2+] in cerebral arteries of rat by membrane potential and intravascular pressure

1. The regulation of intracellular [Ca2+] in the smooth muscle cells in the wall of small pressurized cerebral arteries (100-200 micron) of rat was studied using simultaneous digital fluorescence video imaging of arterial diameter and wall [Ca2+], combined with microelectrode measurements of arterial membrane potential. 2. Elevation of intravascular pressure (from 10 to 100 mmHg) caused a membrane depolarization from -63 +/- 1 to -36 +/- 2 mV, increased arterial wall [Ca2+] from 119 +/- 10 to 245 +/- 9 nM, and constricted the arteries from 208 +/- 10 micron (fully dilated, Ca2+ free) to 116 +/- 7 micron or by 45 % ('myogenic tone'). 3. Pressure-induced increases in arterial wall [Ca2+] and vasoconstriction were blocked by inhibitors of voltage-dependent Ca2+ channels (diltiazem and nisoldipine) or to the same extent by removal of external Ca2+. 4. At a steady pressure (i.e. under isobaric conditions at 60 mmHg), the membrane potential was stable at -45 +/- 1 mV, intracellular [Ca2+] was 190 +/- 10 nM, and arteries were constricted by 41 % (to 115 +/- 7 micron from 196 +/- 8 micron fully dilated). Under this condition of -45 +/- 5 mV at 60 mmHg, the voltage sensitivity of wall [Ca2+] and diameter were 7.5 nM mV-1 and 7.5 micron mV-1, respectively, resulting in a Ca2+ sensitivity of diameter of 1 mum nM-1. 5. Membrane potential depolarization from -58 to -23 mV caused pressurized arteries (to 60 mmHg) to constrict over their entire working range, i.e. from maximally dilated to constricted. This depolarization was associated with an elevation of arterial wall [Ca2+] from 124 +/- 7 to 347 +/- 12 nM. These increases in arterial wall [Ca2+] and vasoconstriction were blocked by L-type voltage-dependent Ca2+ channel inhibitors. 6. The relationship between arterial wall [Ca2+] and membrane potential was not significantly different under isobaric (60 mmHg) and non-isobaric conditions (10-100 mmHg), suggesting that intravascular pressure regulates arterial wall [Ca2+] through changes in membrane potential. 7. The results are consistent with the idea that intravascular pressure causes membrane potential depolarization, which opens voltage-dependent Ca2+ channels, acting as 'voltage sensors', thus increasing Ca2+ entry and arterial wall [Ca2+], which leads to vasoconstriction.

Voltage-dependent K+ currents in smooth muscle cells from mouse gallbladder

The ionic mechanisms associated with the control of gallbladder contractility are incompletely understood. One type of K+ current, the voltage-dependent K+ (KV) current, is relatively uncharacterized in gallbladder cells and may contribute to muscular excitability. The main focus of this study was therefore to determine the voltage dependence and pharmacological nature of this K+ current in isolated myocytes from mouse gallbladder, using the patch-clamp technique. Currents through Ca(2+)-activated K+ channels were minimized by buffering of intracellular Ca2+ (20 nM free Ca2+) and by inclusion of 1 mM tetraethylammonium (TEA+) in the bathing solution. With 140 mM symmetrical K+, membrane depolarization increased K+ currents, independent of driving force, as assessed by tail current analysis. Half-maximal activation of K+ currents occurred at approximately 1 mV and increased e-fold per 9 mV. Inactivation also increased on depolarization, with a midpoint of -24 mV. Single KV channels were recorded in the cell-attached configuration, exhibiting a single-channel conductance of 4.9 pS. TEA+ at 10 mM reduced KV currents by 36%. At +50 mV, 1 mM and 10 mM 4-aminopyridine inhibited currents by 18% and 35%, respectively, whereas 1 and 10 mM 3,4-diaminopyridine inhibited currents by 11% and 21%, respectively. Quinine inhibited KV currents (at +50 mV, 100 microM and 1 mM quinine inhibited current by 24% and 70%, respectively). In summary, we describe voltage-activated K+ currents from the mouse gallbladder that are likely to contribute to the control of muscular excitability.

Voltage-dependent Ca2+ channels in arterial smooth muscle cells

The past years have seen some significant advances in our understanding of the functional and molecular properties of voltage-dependent Ca2+ channels in arterial smooth muscle. Molecular cloning and expression studies together with experiments on native voltage-dependent Ca2+ channels revealed that these channels are built upon a molecular structure with properties appropriate to function as the main source for Ca2+ entry into arterial smooth muscle cells. This Ca2+ entry regulates intracellular free Ca2+, and thereby arterial tone. We summarize several avenues of recent research that should provide significant insights into the functioning of voltage-dependent Ca2+ channels under conditions that occur in arterial smooth muscle. These experiments have identified important features of voltage-dependent Ca2+ channels, including the steep steady-state voltage-dependence of the channel open probability at steady physiological membrane potentials between -60 and -30 mV, and a relatively high permeation rate at physiological Ca2+ concentrations, being about one million Ca2+ ions/s at -50 mV. This calcium permeation rate seems to be a feature of the pore-forming Ca2+ channel alpha1 subunit, since it was identical for native channels and the expressed alpha1 subunit alone. The channel activity is regulated by dihydropyridines, vasoactive hormones and intracellular signaling pathways. While the membrane potential of smooth muscle cells primarily regulates arterial muscle tone through alterations in Ca2+ influx through dihydropyridine-sensitive voltage-dependent ('L-type') Ca2+ channels, the role of these channels in the differentiation and proliferation of vascular smooth muscle cells is less clear. We discuss recent findings suggesting that other Ca2+ permeable ion channels might be important for the control of Ca2+ influx in dedifferentiated vascular smooth muscle cells.

Increased myogenic tone and diminished responsiveness to ATP-sensitive K+ channel openers in cerebral arteries from diabetic rats

Diabetes mellitus has profound adverse effects on vascular and, in particular, endothelial function. Although pressure-induced constriction ("myogenic tone") is a major contributor to the regulation of blood flow, little is known about the effects of diabetes on this response. Diabetes has been shown to diminish the dilation of cerebral arteries to synthetic ATP-sensitive K+ (KATP) channel openers. In this study, we explored the effects of diabetes induced in rats by streptozotocin on cerebral artery (250 to 300 microns) myogenic tone and on vasodilations to the synthetic KATP channel openers pinacidil and levcromakalim. Elevation of intravascular pressure caused a graded membrane potential depolarization and constriction, which was greater in arteries from diabetic rats compared with normal rats (at 60 mm Hg, 5 mV more depolarized and 22 microns more constricted). Pressurized arteries (at 60 mm Hg) from diabetic rats were 5- to 15-fold less sensitive to pinacidil and levcromakalim than were control arteries (EC50 values for pinacidil and levcromakalim were 1.4 and 0.6 mumol/L, respectively, in diabetic animals and 0.3 and 0.04, respectively, in control animals; P < .05). Removal of the endothelium or addition of a NO synthase inhibitor, NG-nitro-L-arginine (LNNA), in control arteries decreased the sensitivity to KATP channel openers and depolarized and constricted control arteries to levels similar to those observed in arteries from diabetic animals. Sodium nitroprusside caused a membrane potential hyperpolarization and enhanced the response to pinacidil in arteries from diabetic animals. Removal of the endothelium or LNNA had little effect on the apparent KATP channel opener sensitivity, the membrane potential, and pressure-induced constrictions of arteries from diabetic animals. The results are consistent with the hypothesis that this type of diabetes leads to a decrease in tonic NO release from the endothelium, which in turn causes membrane potential depolarization and vasoconstriction, resulting in a diminished response to KATP channel openers.

Activators of protein kinase C decrease Ca2+ spark frequency in smooth muscle cells from cerebral arteries

Local Ca2+ transients ("Ca2+ sparks") caused by the opening of one or the coordinated opening of a number of tightly clustered ryanodine-sensitive Ca(2+)-release (RyR) channels in the sarcoplasmic reticulum (SR) activate nearby Ca(2+)-dependent K+ (KCa) channels to cause an outward current [referred to as a "spontaneous transient outward current" (STOC)]. These KCa currents cause membrane potential hyperpolarization of arterial myocytes, which would lead to vasodilation through decreasing Ca2+ entry through voltage-dependent Ca2+ channels. Therefore, modulation of Ca2+ spark frequency should be a means to regulation of KCa channel currents and hence membrane potential. We examined the frequency modulation of Ca2+ sparks and STOCs by activation of protein kinase C (PKC). The PKC activators, phorbol 12-myristate 13-acetate (PMA; 10 nM) and 1,2-dioctanoyl-sn-glycerol (1 microM), decreased Ca2+ spark frequency by 72% and 60%, respectively, and PMA reduced STOC frequency by 83%. PMA also decreased STOC amplitude by 22%, which could be explained by an observed reduction (29%) in KCa channel open probability in the absence of Ca2+ sparks. The reduction in STOC frequency occurred in the presence of an inorganic blocker (Cd2+) of voltage-dependent Ca2+ channels. The reduction in Ca2+ spark frequency did not result from SR Ca2+ depletion, since caffeine-induced Ca2+ transients did not decrease in the presence of PMA. These results suggest that activators of PKC can modulate the frequency of Ca2+ sparks, through an effect on the RyR channel, which would decrease STOC frequency (i.e., KCa channel activity).

ATP-sensitive and inwardly rectifying potassium channels in smooth muscle

The properties and roles of ATP-sensitive (KATP) and inwardly rectifying (KIR) potassium channels are reviewed. Potassium channels regulate the membrane potential of smooth muscle, which controls calcium entry through voltage-dependent calcium channels, and thereby contractility through changes in intracellular calcium. The KATP channel is likely to be composed of members of the inward rectifier channel gene family (Kir6) and sulfonylurea receptor proteins. The KIR channels do not appear to be as widely distributed as KATP channels in smooth muscle and may provide a mechanism by which changes in extracellular K+ can alter smooth muscle membrane potential, and thereby arterial diameter. The KATP channels contribute to the resting membrane conductance of some types of smooth muscle and can open under situations of metabolic compromise. The KATP channels are targets of a wide variety of vasodilators and constrictors, which act, respectively, through adenosine 3',5'-cyclic monophosphate/protein kinase A and protein kinase C. The KATP channels are also activated by a number of synthetic vasodilators (e.g., diazoxide and pinacidil) and are inhibited by the oral hypoglycemic sulfonylurea drugs (e.g., glibenclamide). Together, KATP and KIR channels are important regulators of smooth muscle function and represent important therapeutic targets.

Chloride channel blockers inhibit myogenic tone in rat cerebral arteries

1. We have investigated the role of chloride channels in pressure-induced depolarization and contraction of cerebral artery smooth muscle cells. 2. Two chloride channel blockers, indanyloxyacetic acid (IAA-94) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), caused hyperpolarizations (10-15 mV) and dilatations (up to 90%) of pressurized (80 mmHg), rat posterior cerebral arteries. Niflumic acid, a blocker of calcium-activated chloride channels, did not affect arterial tone. 3. Dilatations to IAA-94 and DIDS were unaffected by potassium channel blockers, but were prevented by elevated potassium. IAA-94 and DIDS had no effect on membrane potential or diameter of arteries at low intravascular pressure, where myogenic tone is absent. Reduction of extracellular chloride (60 mM Cl-) increased the pressure-induced contractions. Removal of extracellular sodium did not affect the pressure-induced responses. 4. Our results suggest that intravascular pressure activates DIDS- and IAA-94-sensitive chloride channels to depolarize arterial smooth muscle, thereby contributing to the myogenic constriction.

Ca(2+)-activated K+ channels regulate action potential repolarization in urinary bladder smooth muscle

The goal of this study was to examine the role of large conductance Ca(2+)-activated K+ channels in the regulation of cell excitability in urinary bladder smooth muscle from the guinea pig. Ca(2+)-activated K+ channels were studied with single-channel recording techniques and found to be intracellular Ca2+ and voltage dependent and sensitive to external tetraethylammonium and blocked by nanomolar concentrations of iberiotoxin (apparent dissociation constant of 4 nM). Spontaneous action potentials recorded from intact tissue strips depended on external Ca2+ and were inhibited by Ca2+ channel blockers. Iberiotoxin (100 nM) significantly altered the configuration of the action potential by increasing the duration and peak amplitude of the action potential and decreasing the rate of decay. Iberiotoxin also increased the action potential frequency from 0.11 to 0.29 Hz. This study suggests that Ca(2+)-activated K+ channels play a significant role in the repolarization of the action potential and in the maintenance of the resting membrane potential of the urinary bladder smooth muscle.

ATP-sensitive potassium channels in cultured arterial segments

Organ cultures of arteries have been used to study growth responses, proliferation, and contractility. However, the function of specific-ion channels in cultured arteries has not been investigated. ATP-sensitive K+ (KATP) channels play an important role in the control of arterial tone. The goal of this study was to determine the functional state of KATP channels in arteries kept in culture. Segments from rabbit mesenteric arteries were cultured in for 2-7 days. To explore the properties of KATP channels, the effects of KATP-channel modulators and other vasoactive substances on isometric force, density, and modulation of KATP currents in single smooth muscle cells isolated from cultured vessels were examined. Isometric contractions were measured with a resistance-vessel myograph. Whole cell KATP currents were recorded with the patch-clamp technique. Membrane capacitance and KATP-current density in single smooth muscle cells from freshly dissected (control) and cultured arteries were not altered. At -60 mV, glibenclamide-sensitive currents in the presence of the K(+)-channel opener pinacidil were -4.7 +/- 1.2, -4.7 +/- 0.6, and -4.6 +/- 0.7 pA/pF for control and 2- and 4-day arteries, respectively. Inhibitory modulation of KATP currents in arterial smooth muscle also remained intact for 4 days in culture; the vasoconstrictor histamine (10 microM) reduced glibenclamide-sensitive currents in the presence of pinacidil by 61.2 +/- 2.8, 42.4 +/- 10.1, and 41.2 +/- 6.1% for control and 2- and 4-day arteries, respectively. Pinacidil relaxed control and cultured arteries (1-7 days) in a dose-dependent manner. Half-maximal effective concentrations of pinacidil were 0.42, 0.24, 0.23, and 0.51 microM for control and 2-, 4-, and 7-day arteries, respectively, whereas maximal relaxations to pinacidil were 62.9, 47.5, 37.5, and 55.7% for control and 2-, 5-, and 7-day arteries, respectively. Histamine, norepinephrine, and serotonin constricted cultured arteries, although responses to histamine and norepinephrine diminished by 30-50% after 5 days in culture. The relaxant effect of acetylcholine was not maintained in cultured arteries. Sodium nitroprusside, however, effectively relaxed arteries cultured for 2-7 days. The data indicate that with the culture model described, KATP channels in arterial smooth muscle remained functional and contractile responses in arterial segments were maintained for up to 7 days. These results suggest that this approach can be used to study either long-term regulation of KATP channels or the role of this channel type in growth responses.

Gender differences in coronary artery diameter involve estrogen, nitric oxide, and Ca(2+)-dependent K+ channels

During their reproductive years, women have a much lower incidence of coronary heart disease compared with men of similar age. Estrogen appears to be largely responsible for this decrease in cardiovascular mortality in women. In the present study, isolated pressurized coronary arteries from rats were used to assess the role of gender and circulating estrogen on coronary vascular function. Pressure-induced constrictions ("myogenic tone") were greater (approximately 2-fold) in isolated coronary arteries from estrogen-deficient male or ovariectomized (OVX) rats compared with similar arteries obtained from female rats or OVX rats receiving physiological levels of estrogen replacement (OVX+E group). These differences in coronary artery diameter were abolished by removal of the vascular endothelium or chemical inhibition of NO synthase. The anti-estrogen, tamoxifen, increased pressure-induced constrictions of coronary arteries from female and OVX+E rats. Dilations of pressurized coronary arteries from female and OVX animals to sodium nitroprusside, a nitrovasodilator that generates NO, were reduced by > 50% by iberiotoxin (IBTX), an inhibitor of Ca(2+)-dependent K+ (KCa) channels. Sodium nitroprusside (10 mumol/L) hyperpolarized coronary arteries by 13 +/- 2 mV, an effect that was greatly diminished (approximately 80%) by IBTX. Coronary arteries isolated from female rats produced greater constrictions in response to IBTX and KT 5823, an inhibitor of cGMP-dependent protein kinase, compared with coronary arteries from OVX rats. cGMP-dependent protein kinase increased the activity of KCa channels 16.5 +/- 5-fold in excised membrane patches from smooth muscle cells enzymatically isolated from these small coronary arteries. We propose that physiological levels of circulating 17 beta-estradiol elevate basal NO release from the endothelial cells, which increases the diameter of pressurized coronary arteries. Further, our results suggest that part of the effect of this NO is through activation of KCa channels in the smooth muscle cells of the coronary arteries.

Vasoconstrictors inhibit ATP-sensitive K+ channels in arterial smooth muscle through protein kinase C

The effects of vasoconstrictor-receptor (neuropeptide Y, alpha-adrenergic, serotonergic, histaminergic) stimulation on currents through ATP-sensitive potassium (KATP) channels in arterial smooth muscle cells were examined. Whole-cell KATP currents, activated by the synthetic KATP channel opener pinacidil or by the endogenous vasodilator, calcitonin gene-related peptide, which acts through protein kinase A, were measured in smooth muscle cells isolated from mesenteric arteries of rabbit. Stimulation of NPY-, alpha 1-, serotonin (5-HT2)-, and histamine (H1)-receptors inhibited KATP currents by 40-56%. The signal transduction pathway that links these receptors to KATP channels was investigated. An inhibitor of phospholipase C (D609) and of protein kinase C (GF 109203X) reduced the inhibitory effect of these vasoconstrictors on KATP currents from 40-56% to 11-23%. Activators of protein kinase C, a diacylglycerol analogue and phorbol 12-myristate 13-acetate (PMA), inhibited KATP currents by 87.3 and 84.2%, respectively. KATP currents, activated by calcitonin gene-related peptide, were also inhibited (47-87%) by serotonin, phenylephrine, and PMA. We propose that KATP channels in these arterial myocytes are subject to dual modulation by protein kinase C (inhibition) and protein kinase A (activation).

Study on inhibition of pancreatic exocrine secretion by somatostatin in rats

We used a potent and specific monoclonal antibody to somatostatin to test the physiologic inhibitory role of the tetradecapeptide somatostatin on pancreatic secretion. Somatostatin immunoneutralization increased both the total amylase and volume of pancreatic secretion. Cholecystokinin-A receptor antagonism abolished the stimulatory effect of somatostatin immunoneutralization. We conclude that somatostatin tonically inhibits, pancreatic secretion in fasted rats via inhibition of the release or action of cholecystokinin. Furthermore, the source of these peptides is likely islet delta cells and intrapancreatic neurons, respectively.

Zeneca ZD6169 activates ATP-sensitive K+ channels in the urinary bladder of the guinea pig

The effects of Zeneca ZD6169, a tertiary carbinol, and levcromakalim were examined on the membrane potential of intact smooth muscle cells, and on ATP-sensitive K+ (KATP) channel currents in isolated smooth muscle cells from the guinea pig urinary bladder. ZD6169 and levcromakalim induced a glibenclamide-sensitive hyperpolarization of the membrane potential. The ZD6169- and levcromakalim-induced KATP currents were half-maximal at 1.02 and 2.63 mumol/l, respectively, with Hill coefficients of 1.46 and 1.62, respectively. The ZD6169-induced KATP currents were inhibited by internal ATP (3.0 mmol/l), reduced 34% by activators of protein kinase C, and decreased 35% when the external pH was lowered to 6.4. This study provides the first characterization of ZD6169 on KATP currents and indicates that ZD6169 is a potent opener of KATP channels in the smooth muscle from the urinary bladder.

Inward rectifier K+ currents in smooth muscle cells from rat coronary arteries: block by Mg2+, Ca2+, and Ba2+

Inward rectifier K+ channels have been implicated in the control of membrane potential and external K(+)-induced dilations of small coronary arteries. To identify and characterize inward rectifier K+ currents in coronary artery smooth muscle, whole cell K+ currents in smooth muscle cells enzymatically isolated from rat coronary (septal) arteries (diameters, 100-150 microns) were measured in the conventional and perforated configurations of the patch-clamp technique. Ba(2+)-sensitive, whole cell K+ current-voltage relationships exhibited inward rectification. Blockers of Ca(2+)-activated K+ channels (1 mM tetraethylammonium ion), ATP-sensitive K+ channels (10 microM glibenclamide), and voltage-dependent K+ channels (1 mM 4-aminopyridine) in smooth muscle did not affect inward rectifier K+ currents. The nonselective K+ channel inhibitor phencyclidine (100 microM) reduced inward rectifier K+ currents by approximately 50%. External Ba2+ reduced inward currents, with membrane potential hyperpolarization increasing inhibition. The half-inhibition constant for Ba2+ was 2.1 microM at -60 mV, decreasing e-fold for a 25-mV hyperpolarization. External Cs+ also blocked inward rectifier K+ currents, with the half-inhibition constant for Cs+ of 2.9 mM at -60 mV. External Ca2+ and Mg2+ reduced inward rectifier K+ currents. At -60 mV, Ca2+ and Mg2+ (1 mM) reduced inward currents by 33 and 21%, respectively. Inward rectification was not affected by dialysis of the cell's interior with a nominally Ca(2+)- and Mg(2+)-free solution. These findings indicate that inward rectifier K+ channels exist in coronary artery smooth muscle and that Ba2+ may be a useful probe for the functional role of inward rectifier K+ channels in coronary arteries.

Rat exocrine pancreatic secretion by vagal stimulation occurs via multiple mediators

The vagus is a mixed nerve containing cholinerrgic and non-cholinergic neurons. Vagal fibers interact with peptidergic neurons of the enteric nervous system which stain immunohistochemically for cholecystokinin, vasoactive intestinal polypeptide, and gastrin releasing peptide. The contribution of these peptidergic neurons in the pancreatic response to vagal stimulation is unknown. We tested the effect of specific inhibitor of these stimulants against vagally mediated exocrine secretion in rats. The response to vagal stimulation was blocked significantly by each of the following: the ganglionic blocker hexamethonium (100% inhibition); the muscarinic, cholinergic blocker atropine (85% inhibition); the specific cholecystokinin-A receptor blocker (91% inhibition); and a vasoactive intestinal polypeptide polyclonal antibody (89% inhibition). This observation is consistent with the hypothesis that potentiating interactions among several agonists mediate the vagal response. Our study, however, dose not exclude acetylcholine as the final common mediator.

Studies of the K(ATP) channel opening activity of the new dihydropyridine compound 9-(3-cyanophenyl)-3,4,6,7,9,10-hexahydro-1,8-(2H,5H)-acridined ione in bladder detrusor in vitro

The potassium (K+) channel opening activity of ZM244085 (9-(3-cyanophenyl)-3,4,6,7,9,10-hexahydro-1,8-(2H,5H)-acridined ione, CAS 149398-59-4), a novel dihydropyridine (DHP), was ascertained. In a set of functional assays, its mechanoinhibitory effect on myogenic activity of guinea pig bladder detrusor muscles, either mildly or highly depolarized with 15 or 80 mmol/l KCl, was measured. ZM244085 had negligible effect on the tone of the detrusor contracted with 80 mmol/l KCl but reduced the myogenic activity induced with 15 mmol/l KCl (IC50=4.2 +/- 0.4 mumol/l). Glibenclamide, an ATP-sensitive K+ (KATP) channel blocker, competitively antagonized this action of ZM244085 with a pA2 value of 7.6. This functional profile of ZM244085 is similar to that of the prototypic K+ channel opener cromakalim but stands in contrast to that of typical DHP Ca2+ channel blockers such as nifedipine and nimodipine. The membrane potential of the guinea pig detrusor, recorded with intracellular microelectrodes, was hyperpolarized 6.8 +/- 3.1 mV by ZM244085 (10 mumol/l). This hyperpolarization was completely blocked by glibenclamide but not affected by apamin (10 mumol/l), a toxin blocking specifically small conductance and Ca2+ dependent K+ (SKCa) channels. ZM244085 (10 mumol/l) increased the whole cell KATP current in isolated guinea pig detrusor cells by 8.8 +/- 2.5 pA, but failed to activate large conductance and Ca2+ dependent K+ (BKCa) channels in excised inside-out membrane patches from those cells. The results from these studies showed that ZM244085 is a K+ channel opener which activates predominantly KATP channels in vitro to relax bladder detrusors.

Extracellular K(+)-induced hyperpolarizations and dilatations of rat coronary and cerebral arteries involve inward rectifier K(+) channels

1. The hypothesis that inward rectifier K(+) channels are involved in the vasodilatation of small coronary and cerebral arteries (100-200 microm diameter) in response to elevated [K+]o was tested. The diameters and membrane potentials of pressurized arteries from rat were measured using a video-imaging system and conventional microelectrodes, respectively. 2. Elevation of [K+]o from 6 to 16 mM caused the membrane potential of pressurized (60 mmHg) arteries to hyperpolarize by 12-14 mV. Extracellular Ba(2+) (Ba2+(o)) blocked K(+)-induced membrane potential hyperpolarizations at concentrations (IC(50), 6 microM) that block inward rectifier K(+) currents in smooth muscle cells isolated from these arteries. 3. Elevation of [K+]o from 6 to 16 mM caused sustained dilatations of pressurized coronary and cerebral arteries with diameters increasing from 125 to 192 microm and 110 to 180 microm in coronary and cerebral arteries, respectively. Ba2+(o) blocked K(+)-induced dilatations of pressurized coronary and cerebral arteries (IC50, 3-8 microM). 4. Elevated [K+]o-induced vasodilatation was not prevented by blockers of other types of K(+) channels (1 mM 4-aminopyridine, 1 mM TEA+, and 10 mu M glibenclamide), and blockers of Na(+)-K(+)-ATPase. Elevated [K+]o-induced vasodilatation was unaffected by removal of the endothelium. 5. These findings suggest that K+(o) dilates small rat coronary and cerebral arteries through activation of inward rectifier K(+) channels. Furthermore, these results support the hypothesis that inward rectifier K(+) channels may be involved in metabolic regulation of coronary and cerebral blood flow in response to changes in [K+]o.