Genomewide association and identification of candidate genes for ovulation rate in swine.

Reproductive efficiency has a great impact on the economic success of pork production. Ovulation rate is an early component of reproduction efficiency and contributes to the number of pigs born in a litter. To better understand the underlying genetics of ovulation rate, a genomewide association study was undertaken. Samples of DNA were collected and tested using the Illumina Porcine SNP60 BeadChip from 1,180 females with ovulation measurements ranging from never farrowed to measurements taken after parity 2. A total of 41,848 SNP were tested using the Bayes C option of GenSel. After the Bayes C analysis, SNP were assigned to sliding windows of 5 consecutive SNP by chromosome-position order beginning with the first 5 SNP on SSC1 and ending with the last 5 SNP on SSCX. The 5-SNP windows were analyzed using the Predict option of GenSel. From the Predict analysis, putative QTL were selected having no overlap with other 5-SNP window groups, no overlap across chromosomes, and the highest genetic variation. These putative QTL were submitted to statistical testing using the bootstrap option of GenSel. Of the putative QTL tested, 80 were found to be statistically significant (P < 0.01). Ten QTL were found on SSC1, 12 on SSC2, 4 on SSC3, 8 on SSC4, 3 on SSC5, 3 on SSC6, 3 on SSC7, 4 on SSC8, 2 on SSC9, 4 on SSC10, 1 on SSC12, 4 on SSC13, 2 on SSC14, 4 on SSC15, 4 on SSC16, 6 on SSC17, 4 on SSC18, and 1 on SSCX. Sixteen QTL were found to be statistically significant at the P < 0.001 level. Six additional QTL were significant at the P = 0.001 level. These 22 QTL accounted for 71.10% of the total genetic variance. The most compelling candidate genes in these regions include Estrogen receptor 1, growth differentiation factor 9, and inhibin ßA. These QTL, when combined with information on genes found in the same regions, should provide useful information that could be used for marker assisted selection, marker assisted management, or genomic selection applications in commercial pig populations.

Identifying genetic loci controlling neonatal passive transfer of immunity using a hybrid genotyping strategy.

Colostrum intake is critical to a piglet's survival and can be measured by precipitating out the γ-immunoglobulins from serum with ammonium sulfate (immunocrit). Genetic analysis of immunocrits on 5312 piglets indicated that the heritabilities (se) for direct and maternal effects were 0.13 (0.06) and 0.53 (0.08) respectively. To identify QTL for direct genetic effects, piglets with the highest and lowest immunocrits from 470 litters were selected. Six sets of DNA pools were created based on sire of the litter. These 12 DNA pools were applied to Illumina Porcine SNP60 BeadChips. Normalized X and Y values were analyzed. Three different SNP selection methods were used: deviation of the mean from high vs. low pools, the deviation adjusted for variance based on binomial theory and ANOVA. The 25 highest ranking SNPs were selected from each evaluation for further study along with 12 regions selected based on a five-SNP window approach. Selected SNPs were individually genotyped in the 988 piglets included in pools as well as in 524 piglets that had intermediate immunocrits. Association analyses were conducted fitting an animal model using the estimated genetic parameters. Nineteen SNPs were nominally associated (P < 0.01) with immunocrit values, of which nine remained significant (P < 0.05) after Bonferroni correction, located in 16 genomic regions on 13 chromosomes. In conclusion, the pooling strategy reduced the cost to scan the genome by more than 80% and identified genomic regions associated with a piglet's ability to acquire γ-immunoglobulin from colostrum. Each method to rank SNPs from the pooled analyses contributed unique validated markers, suggesting that multiple analyses will reveal more QTL than a single analysis.

Genome-wide association of meat quality traits and tenderness in swine.

Pork quality has a large impact on consumer preference and perception of eating quality. A genome-wide association was performed for pork quality traits [intramuscular fat (IMF)], slice shear force (SSF), color attributes, purge, cooking loss, and pH] from 531 to 1,237 records on barrows and gilts of a Landrace-Duroc-Yorkshire population using the Illumina PorcineSNP60 BeadChip. Associations were detected using MTDFREML for all traits. Intramuscular fat had the greatest number of SNP associations, followed by pH, purge, cooking loss, shear force, and color. Two regions contained associations for multiple traits; one on SSC1 at 255 Mb near calcineurin subunit B (PPP3R2) was associated with SSF, moisture loss, and pH, and one on SSC6 from 28 to 29.5 Mb for purge and IMF containing the candidate genes glucose-6-phosphate isomerase (GPI) and KCTD15. Some of the other compelling candidate genes in regions associated with meat quality include CEBPA, SNAI1, and FAM132A for IMF, CAPN1 for SSF, GLUL for pH, and PRKAG3 and ITGB1 with cooking loss.

Genome-wide association study of swine farrowing traits. Part II: Bayesian analysis of marker data.

Reproductive efficiency has a great impact on the economic success of pork (sus scrofa) production. Number born alive (NBA) and average piglet birth weight (ABW) contribute greatly to reproductive efficiency. To better understand the underlying genetics of birth traits, a genome-wide association study (GWAS) was undertaken. Samples of DNA were collected and tested using the Illumina PorcineSNP60 BeadChip from 1,152 first parity gilts. Traits included total number born (TNB), NBA, number born dead (NBD), number stillborn (NSB), number of mummies (MUM), total litter birth weight (LBW), and ABW. A total of 41,151 SNP were tested using a Bayesian approach. Beginning with the first 5 SNP on SSC1 and ending with the last 5 SNP on the SSCX, SNP were assigned to groups of 5 consecutive SNP by chromosome-position order and analyzed again using a Bayesian approach. From that analysis, 5-SNP groups were selected having no overlap with another 5-SNP groups and no overlap across chromosomes. These selected 5-SNP non-overlapping groups were defined as QTL. Of the available 8,814 QTL, 124 were found to be statistically significant (P < 0.01). Multiple testing was considered using the probability of false positives. Eleven QTL were found for TNB, 3 on SSC1, 3 on SSC4, 1 on SSC13, 1 on SSC14, 2 on SSC15, and 1 on SSC17. Statistical testing for NBA identified 14 QTL, 4 on SSC1, 1 on SSC4, 1 on SSC6, 1 on SSC10, 1on SSC13, 3 on SSC15, and 3 on SSC17. A single NBD QTL was found on SSC11. No QTL were identified for NSB or MUM. Thirty-three QTL were found for LBW, 3 on SSC1, 1 on SSC2, 1 on SSC3, 5 on SSC4, 2 on SSC5, 5 on SSC6, 3 on SSC7, 2 on SSC9, 1 on SSC10, 2 on SSC14, 6 on SSC15, and 2 on SSC17. A total of 65 QTL were found for ABW, 9 on SSC1, 3 on SSC2, 9 on SSC5, 5 on SSC6, 1 on SSC7, 2 on SSC8, 2 on SSC9, 3 on SSC10, 1 on SSC11, 3 on SSC12, 2 on SSC13, 8 on SSC14, 8 on SSC15, 1 on SSC17, and 8 on SSC18. Several candidate genes have been identified that overlap QTL locations among TNB, NBA, NBD, and ABW. These QTL when combined with information on genes found in the same regions should provide useful information that could be used for marker assisted selection, marker assisted management, or genomic selection applications in commercial pig populations.

MicroRNA expression profile in bovine cumulus-oocyte complexes: possible role of let-7 and miR-106a in the development of bovine oocytes.

The objectives of this study included: (1) identify the expression of miRNAs specific to bovine cumulus-oocyte complexes (COCs) during late oogenesis, (2) characterize the expression of candidate miRNAs as well as some miRNA processing genes, and (3) computationally identify and characterize the expression of target mRNAs for candidate miRNAs. Small RNAs in the 16-27 bp range were isolated from pooled COCs aspirated from 1- to 10-mm follicles of beef cattle ovaries and used to construct a cDNA library. A total 1798 putative miRNA sequences from the cDNA library of small RNA were compared to known miRNAs. Sixty-four miRNA clusters matched previously reported sequences in the miRBase database and 5 miRNA clusters had not been reported. TaqMan miRNA assays were used to confirm the expression of let-7b, let-7i, and miR-106a from independent collections of COCs. Real-time PCR assays were used to characterize expression of miRNA processing genes and target mRNAs (MYC and WEE1A) for the candidate miRNAs from independent collections of COCs. Expression data were analyzed using general linear model procedures for analysis of variance. The expression of let-7b and let-7i were not different between the cellular populations from various sized follicles. However, miR-106a expression was greater (P<0.01) in oocytes compared with COCs and granulosa cells. Furthermore, all the miRNA processing genes have greater expression (P<0.001) in oocytes compared with COCs and granulosa cells. The expression of potential target mRNAs for let-7 and let-7i (i.e., MYC), and miR-106a (i.e., WEE1A) were decreased (P<0.05) in oocytes compared with COCs and granulosa cells. These results demonstrate specific miRNAs within bovine COCs during late oogenesis and provide some evidence that miRNAs may play a role regulating maternal mRNAs in bovine oocytes.

Platelet glycoprotein IIb/IIIa receptor inhibitor preserves coronary flow reserve during progressive coronary arteriostenosis in swine.

Thrombosis resulting from blood platelet aggregation via glycoprotein (GP) IIb/IIIa receptor activation triggers the local release of vasoactive substances. Therefore, inhibition of these receptors could affect coronary vasoactive function during thrombotic coronary arteriostenosis. Twenty pigs were instrumented with an aortic catheter and with hydraulic occluders and flow probes on both the left anterior descending (LAD) and the left circumflex (LCx) coronary arteries. One of these 2 coronary arteries was repeatedly injured by external clamping for 15-second periods at 30-minute intervals while the pigs were given either a GP IIb/IIIa receptor inhibitor (L-739,758) (n=5), heparin (n=5), aspirin (n=3), or saline (n=7). There were no baseline differences between the 4 groups in mean arterial pressure, resting coronary blood flow (CBF), or reactive hyperemic response (RHR), which was induced by brief coronary artery occlusion and expressed as flow debt repayment. After multiple injuries, resting CBF had decreased by 95+/-2% (ie, nearly complete coronary artery occlusion) at 15+/-4 minutes in the control group, whereas in the heparin-, aspirin-, and GP IIb/IIIa inhibitor-treated groups, resting CBF had decreased by only 21+/-7% at 18+/-3 minutes, 15+/-3% at 18+/-5 minutes, and 15+/-7% at 21+/-4 minutes, respectively, suggesting that heparin, aspirin, and the GP IIb/IIIa inhibitor each prevented injury-induced coronary artery occlusion. After the initial injury, the RHR was progressively reduced in the control and heparin- and aspirin-treated groups but not in the GP IIb/IIIa inhibitor-treated group. At a comparable level of resting CBF ( approximately 15% below baseline), the RHR was reduced more in the control (-56+/-9%), heparin-treated (-49+/-9%), and aspirin-treated (-61+/-12) groups (P:<0.05) than in the GP IIb/IIIa inhibitor-treated group (-26+/-6%). When the resting CBF had decreased by approximately 35%, the RHR still was reduced significantly more (P<0.01) in the heparin-treated group (-64+/-9%) than in the GP IIb/IIIa inhibitor-treated group (-21+/-6%). In a separate group of control pigs (n=4) subjected to 2 injuries, coronary perfusion pressure distal to the injury site was reduced by 14+/-1 mm Hg from the arterial pressure, and the RHR was 20+/-6%. When the distal coronary perfusion pressure was reduced similarly (-14+/-1 mm Hg) in a separate group of GP IIb/IIIa inhibitor-treated pigs (n=4) by 2 injuries and the use of a hydraulic occluder, the RHR was 130+/-16% (P<0.01 versus control). Our data demonstrate for the first time that a platelet GP IIb/IIIa receptor inhibitor can preserve the distal coronary vasodilatory response during progressive coronary arteriostenosis.

A novel heart failure model induced by sequential coronary artery occlusions and tachycardiac stress in awake pigs.

A heart failure model was developed using conscious pigs subjected to serial myocardial infarctions followed by intermittent rapid ventricular pacing. Aortic and atrial catheters, left ventricular (LV) pressure gauge, LV dimension crystals, ascending aortic flow probe, pacing leads, and two coronary artery occluders were implanted in 15 pigs. The initial distal left circumflex coronary artery (LCX) occlusion produced a modest infarct, i.e., 18 +/- 3% of LV, and the second proximal LCX occlusion, performed 48 h later, enlarged the infarct to 33 +/- 2% of the LV with only modest changes in LV function. Thereafter, the pigs were subjected to ventricular pacing at 220 beats/min, which was maintained for 7 days and terminated for 3 days. This pacing cycle was repeated two more times and resulted in significantly impaired LV function and systemic hemodynamics. For example, after the second cycle of pacing, LV rate of pressure change (dP/dt, -41 +/- 4% from 2,778 +/- 112 mmHg/s), velocity of circumferential fiber shortening (V(cf): -53 +/- 6% from 1.1 +/- 0.1 s(-1)), and cardiac index (CI: -42 +/- 5% from 122 +/- 4 ml. min(-1). kg(-1)) were reduced significantly, whereas LV end-diastolic diameter (EDD: +34 +/- 6% from 39 +/- 2 mm), total peripheral resistance (TPR: +75 +/- 16% from 0.79 +/- 0.05 U), and mean left atrial pressure (LAP) (+21 +/- 1 mmHg from 5 +/- 1 mmHg) were increased significantly. Importantly, 3 wk after cessation of the final pacing cycle, LV dP/dt (-40 +/- 5%), V(cf) (-48 +/- 9%), and CI (-30 +/- 4%) remained depressed, whereas LV EDD (+39 +/- 5%), TPR (+43 +/- 9%), and LAP (+13 +/- 4 mmHg) were still increased. In contrast, hemodynamic impairment in six conscious pigs subjected to pacing only did not persist when pacing was terminated. Thus this model could provide a unique opportunity to study both the effects of preclinical therapeutic interventions and the mechanisms involved in the development of heart failure.

Combined effects of angiotensin converting enzyme inhibition and angiotensin II receptor antagonism in conscious pigs with congestive heart failure.

OBJECTIVE: The goal of this study was to determine if the hemodynamic effects of the combined administration of an angiotensin converting enzyme (ACE) inhibitor and angiotensin II type 1 (AT1) receptor antagonist are greater than those produced by either of these agents administered individually during heart failure. METHODS: Ten farm pigs were chronically instrumented with aortic, left atrial and right atrial catheters, a left ventricular (LV) pressure gauge, LV dimension crystals, coronary occluders, an ascending aortic flow probe and pacing leads. Heart failure was induced by serial myocardial infarctions followed by repeated rapid ventricular pacing. RESULTS: Heart failure was manifested by significant (p < 0.01) decreases in LV dP/dt (-38 +/- 5%, from 2943 +/- 107 mmHg/s) and cardiac output (-27 +/- 4%, from 4.1 +/- 0.2 l/min) and increases in left atrial pressure (+18 +/- 1 mmHg, from 4 +/- 1 mmHg) and total peripheral resistance (TPR)(+40 +/- 8%, from 23 +/- 2 mmHg/l/min). The effects of an ACE inhibitor (enalaprilat) and an AT1 receptor antagonist (L-158,809), administered in maximally effective doses, either individually or concomitantly, were examined on different days in conscious pigs with heart failure. There were no differences in any of the baseline hemodynamic measurements among the groups studied. Thirty minutes after administration, enalaprilat (4 mg/kg i.v.) increased (p < 0.05) cardiac output by 8 +/- 2% and reduced (p < 0.05) mean arterial pressure and TPR by 5 +/- 1 and 12 +/- 1%, respectively, while the changes in LV dP/dt (0 +/- 2%), LV fractional shortening (+4 +/- 3%) and heart rate (+1 +/- 1%) were not statistically significant. Similarly, L-158,809 (4 mg/kg, i.v.) increased cardiac output by 9 +/- 2% and reduced mean arterial pressure and TPR by 4 +/- 1 and 11 +/- 3%, respectively, while the changes in LV dP/dt (+3 +/- 3%), LV fractional shortening (+3 +/- 1%) and heart rate (0 +/- 1%) were not significant. However, enalaprilat (1 mg/kg, i.v.) and L-158,809 (1 mg/kg, i.v.), administered concomitantly, reduced TPR by 21 +/- 3%, an effect greater (p < 0.05) than when either of these agents was administered individually at a dose of 4 mg/kg, i.v. The changes in mean arterial pressure (-9 +/- 2%), cardiac output (+15 +/- 4%) and LV fractional shortening (+11 +/- 3%) also tended to be greater with concomitant administration. In addition, in a sequential dosing protocol, when L-158,809 (1 mg/kg, i.v.) was administered 30 min after enalaprilat (1 mg/kg, i.v.), TPR was reduced by 20 +/- 4% compared to only a 6 +/- 3% reduction (p < 0.05) when the enalaprilat was followed 30 min later by a second dose of enalaprilat (1 mg/kg, i.v.). The changes in mean arterial pressure and cardiac output for the combined treatment group also tended to be greater than those for the group given two sequential doses of enalaprilat. CONCLUSION: In conscious pigs with heart failure, the combined vasodilatory effects of an ACE inhibitor and AT1 receptor antagonist are greater than those produced when only one of these agents is administered, suggesting that independent mechanisms of ACE inhibition and AT1 receptor antagonism could be partly responsible for the improved vascular dynamics during heart failure.

GH replacement fails to improve ventricular function in hypophysectomized rats with myocardial infarction.

To determine whether growth hormone (GH) replacement improves cardiac function, GH-deficient hypophysectomized rats with moderate myocardial infarction (MI) were studied after 3 wk of treatment with either recombinant rat GH (3.2 mg.kg-1.day-1 sc) or vehicle. The serum insulin-like growth factor I level in rats after GH treatment was approximately 10-fold greater than in vehicle-treated rats. GH replacement prevented a decrease in body weight at 1 wk (+5 +/- 6 vs. -26 +/- 4 g in vehicle group, P < 0.01) and increased body weight at 3 wk (+40 +/- 5 vs. -30 +/- 4 g in vehicle group, P < 0.01) after MI. Infarct size, expressed as a percentage of left ventricular (LV) perimeter, was similar for GH-treated (21 +/- 3%) and vehicle-treated (23 +/- 3%) rats. Basal LV systolic pressure, LV end-diastolic pressure, LV dP/dt, mean arterial pressure and heart rate, and the changes in these parameters in response to isoproterenol and norepinephrine were similar for these two groups. Although GH replacement tended to prevent depression in myocardial contractility during the recovery period after maximal stimulation either by the largest dose of isoproterenol (0.8 microgram/kg iv) or by acute volume loading, differences between the two groups were not statistically significant. In addition, to determine the effects of excess GH treatment in a severe state of cardiac dysfunction, nonhypophysectomized rats with larger infarcts (i.e., > 45% of the LV) were studied after 4 wk of treatment. There were no differences either in hemodynamic indexes or in infarct size between the GH- and vehicle-treated groups, whereas body weight had increased (P < 0.01) in the GH-treated group. Thus, although GH treatment effectively prevents the loss of body weight after MI, neither GH replacement nor excess GH treatment plays an important role in preserving cardiac function in rats with moderate or large MI.

Dissociation of phospholamban regulation of cardiac sarcoplasmic reticulum Ca2+ATPase by quercetin.

Quercetin had a biphasic effect on Ca2+ uptake and calcium-stimulated ATP hydrolysis in isolated cardiac sarcoplasmic reticulum (SR). Stimulation of Ca2+ATPase was observed at low quercetin concentrations (<25 microM) followed by inhibition at higher concentrations. The effects were dependent upon the SR protein concentration, the MgATP concentration, and intact phospholamban regulation of cardiac Ca2+ATPase. Only the inhibitory effects at higher quercetin concentrations were observed in skeletal muscle SR which lacks phospholamban and in cardiac SR treated to remove phospholamban regulation. Stimulation was additive with monoclonal antibody 1D11 (directed against phospholamban) at submaximal antibody concentrations; however, the maximal antibody and quercetin stimulation were identical. Quercetin increased the calcium sensitivity of the Ca2+ATPase like that observed with phosphorylation of phospholamban or treatment with monoclonal antibody 1D11. In addition, low concentrations of quercetin increased the steady-state formation of phosphoenzyme from ATP or Pi, but higher quercetin decreased phosphoenzyme levels. Quercetin, even under stimulatory conditions, was a competitive inhibitor of ATP, but appears to relieve the Ca2+ATPase from phospholamban inhibition, thereby, producing an activation. The subsequent inhibitory action of higher quercetin concentrations results from competition of quercetin with the nucleotide binding site of the Ca2+ATPase. The data suggest that quercetin interacts with the nucleotide binding site to mask phospholamban's inhibition of the SR Ca2+ATPase and suggests that phospholamban may interact at or near the nucleotide binding site.

Discontinuous conduction at Purkinje-ventricular muscle junction.

Conduction through the cardiac syncytium varies from being nearly continuous, with very well coupled cells, to being clearly discontinuous, with significant conduction delays over very short distances. The Purkinje-ventricular muscle junction (PVJ) sites on the endocardial surface have characteristic delays of conduction and the presence of discrete groups of cells that suggest significant discontinuities of the conduction process at PVJ sites, as compared with the more nearly continuous conduction within either the Purkinje or the ventricular muscle layers of the papillary muscle. The purpose of the present study was to examine the relative sensitivity of conduction at PVJ sites versus conduction within the Purkinje or the ventricular muscle layer of the canine papillary muscle to agents that modulate L-type calcium current. We have used cadmium as a relatively specific blocker of L-type calcium current and isoproterenol as an agent to increase L-type calcium current to test the hypothesis that discontinuous conduction at the PVJ sites would be more sensitive to these agents than would continuous conduction within either the Purkinje layer or the ventricular muscle layer of a canine papillary muscle. Conduction delay at the PVJ sites was significantly increased by cadmium, with some PVJ sites reversibly becoming nonjunctional at 200-400 microM cadmium. Isoproterenol significantly decreased PVJ delay, and this effect was attenuated by carbachol. All of the effects on conduction delay at the PVJ sites were much greater than the effects for the same agents on conduction velocity within either the Purkinje or the ventricular muscle layer of the papillary muscle.

Myocardial contractile behavior of a new sotalol derivative.

The effects of E4031, a new class III antiarrhythmic agent similar to sotalol, were tested in isometrically contracting rabbit papillary muscles and in anesthetized, open-chest dogs. In papillary muscles, E4031 caused a modest dose-dependent increase of 26 +/- 8% in developed tension and 38 +/- 8% in its maximal rate of rise. Since there was no significant change in the maximal rate of relaxation, the ratio between both maximal velocities increased from 0.92 +/- 0.03 to 1.19 +/- 0.10. Time to peak tension did not change significantly, whereas time to half relaxation increased from 72 +/- 3 to 85 +/- 4 ms. The effective refractory period in the rabbit papillary muscles increased from 179 +/- 10 to 414 +/- 45 ms. In the open-chest dog, the i.v. administration of E4031 did not induce significant changes in heart rate, mean arterial pressure, or left ventricular end diastolic pressure. +dP/dt increased from 1,839 +/- 162 to 2,470 +/- 247 mm Hg/s with no significant change in -dP/dt after 100 micrograms/kg of E4031. Consequently, (+dP/dt)/(-dP/dt) increased from 0.97 +/- 0.07 to 1.18 +/- 0.08. To further evaluate the effects of E4031 on myocardial relaxation, the time constant of isovolumic left ventricular pressure decay was measured by two different methods (tau 1 and tau 2) before and after administering 10 micrograms/kg E4031. Tau 1 increased from 27 +/- 1.8 to 33 +/- 1.6 ms and tau 2 increased from 30 +/- 2.3 to 41 +/- 3.3 ms.(ABSTRACT TRUNCATED AT 250 WORDS)

Negative lusitropic effect of DPI 201-106 and E4031. Possible role of prolonging action potential duration.

In open-chest anesthetized dogs, the time constant of isovolumic left ventricular pressure decay increased following the intravenous administration of either E4031, a class III antiarrhythmic agent which acts by K+ channel blockade, or DPI 201-106 (DPI), a cardiotonic agent which acts by delaying Na+ channel inactivation. In addition to prolonging cardiac refractoriness, both E4031 and DPI increased left ventricular +dP/dt but without significantly altering -dP/dt. Consequently, the value of the ratio (+dP/dt)/(-dP/dt) increased. There were no significant changes in heart rate, mean arterial pressure, or left ventricular end diastolic pressure. Since both E4031 and DPI prolonged the action potential duration (APD) and the refractory period, and slowed relaxation in vivo, the possibility of a causal link between these effects was further investigated under in vitro conditions. In isometrically contracting rabbit papillary muscles, E4031 and DPI increased peak developed tension (DT) and its maximal rate of rise (+T). Since the maximal rate of fall of DT (-T) did not increase by the same factor that +T increased, the value of the ratio +T/-T increased. Time to half relaxation increased, whereas time to peak tension was not significantly changed by either E4031 or DPI. These negative lusitropic effects produced by E4031 or DPI were not observed when equivalent increases in contractility were produced by increasing the extracellular Ca2+ concentration. The effective refractory period measured in the papillary muscles increased following superfusion with either of the two drugs, consistent with their known ability to increase APD. A causal link between the prolongation of APD and the negative lusitropic effects of E4031 and DPI is postulated as the possible mechanism.

Effects of hypoxia, hyperkalemia, and metabolic acidosis on canine subendocardial action potential conduction.

We have studied the individual and combined effects of elevated external potassium concentration (8 mM [K+], metabolic acidosis (pH = 6.8), and hypoxia at different stimulation 400 milliseconds) on Purkinje (P) and ventricular (V) conduction velocities and on Purkinje-ventricular junctional conduction delay (PVJ delay) in in vitro preparations from canine ventricles. Elevated [K+] had opposite effects on P and V velocities, increasing V velocity by 8% while reducing P velocity by 7%. Acidosis reduced P velocity by 9% while reducing V velocity by only 4%. Hypoxia and rapid stimulation rates had no significant effect on either P or V velocities. All test solutions (except hypoxia alone) significantly increased the PVJ delay. The magnitude of the increase in PVJ delay was much greater than the effects on either P or V velocity. In addition, hypoxia and rapid stimulation augmented the increase in PVJ delay in the presence of elevated [K+] and/or acidosis. The special features of conduction at the PV junctional sites may produce altered pathways of excitation of the ventricles during myocardial ischemia.