Browsing by Subject "microdialysis"
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Item Leucine-Enkephalin and Sympathetic Control of Heart Rate(2001-12-01) Stanfill, Amber; Caffrey, James L.; Downey, H. Fred; Shi, XiangrongStanfill, Amber A., Leucine-enkephalin and Sympathetic Control of Heart Rate. Master of Science (Biomedical Sciences), December, 2001, 51 pp., 1 table, 4 figures, references, 48 titles. The following study examined the role of leucine-enkephalin in the sympathetic regulation of the cardiac pacemaker. Leucine-enkephalin (0.3 mM) was administered, by microdialysis into the interstitium of the sinoatrial node in 10 mongrel dogs in conjunction with either sympathetic nerve stimulation or infused norepinephrine. In study one, the right cardiac sympathetic nerves were isolated as they exit the stellate ganglion and stimulated to produce graded (low, 20-30; high 40-50 bpm) increases in heart rate. Once stimulation frequencies were determined, leucine-enkephalin (0.3mM) was added to the dialysis inflow and perfused at 5: 1/min thereafter. The sympathetic stimulations were repeated after 5 and 20 min exposure to leucine-enkephalin. The resulting increases in heart rate during sympathetic stimulation were attenuated at both low (18.2 ±1.3 to 11.4 ±1.4 bpm) and high (45 ±1.5 to 22.8 ±1.5 bpm) frequency stimulation. The degree of inhibition was nearly identical after 20 minutes exposure providing no evidence for a progressively evolving response and for desensitization. Vagal function was also evaluated at 5 and 20 min by stimulating the right cervical vagus at 1 and 3 Hz. Leucine-enkephalin reduced the vagal bradycardia approximately 50% at both time intervals. The administration of the delta-selective opioid antagonist, naltrindole, restored only one third of the sympathetically medicated tachycardia. The same dose of naltrindole completely reversed the coincident vagolytic of leucine-enkephalin. These observations suggested that the sympatholytic effect was either non-opioid or mediated by a different opioid receptor subtype. Study two was conducted to determine if the sympatholytic effect was prejunctional and post-junctional in character. Norepinephrine was added to the dialysis inflow into the SA node in a concentration (6-9 μM) sufficient to produce an intermediate increase in heart rate. The average increase in heart rate was 35.2 ±1.8 bpm. Leucine-enkephalin was then combined with norepinephrine and sympathetic and parasympathetic responses were recorded at 5-min intervals for 20 minutes. The tachycardia mediated by added norepinephrine was unaltered by leucine-enkephalin or the subsequent addition of naltrindole. At the same time intervals, vagal control of heart rate was reduced by more than 50% and then completely restored by naltrindole. When combined with observations in study one, the data support the conclusion that the local nodal sympatholytic effect of leucine-enkephalin was the result of a reduction in the effective interstitial concentration of norepinephrine and not the result of a post-junctional interaction between leucine-enkephalin and norepinephrine.Item Local Enkephalins Modulate Vagal Control of Heart Rate(2001-05-01) Jackson, Keith E.; James L. Caffrey; H. Fred Downey; Michael W. MartinJackson, Keith E., Local Enkephalins Modulate Vagal Control of Heart Rate. Doctor of Philosophy (Biomedical Sciences), May 2001; 112pp; 7 tables; 22 figures; bibliography, 99 titles. Endogenous opioids, such as enkephalins, were first investigated for their ability to modulate pain. A body of evidence now supports opioid actions in many facets of regulation, including the cardiovascular system. Our laboratory is particularly interested in the ability of opioids to modulate autonomic function. Specifically, the role of the endogenous encephalin, methionine-enkephalin-arginine-phenylalanine (MEAP) was investigated to determine its ability to modulate parasympathetic function in the canine. To investigate MEAP’s response in the sinoatrial (SA) node a novel application of microdialysis was employed, whereby microdialysis was employed, whereby microdialysis probes were fabricated as described by Dr. David Van Wylen (38), and implanted in the SA node. After implantation of the probe, there was a significant attenuation of vagal function during the nodal application of MEAP. Specifically, vagally mediated bradcardia was reduced as compared to control, during the nodal application of MEAP. This inhibition of the vagus by MEAP was blocked by naltrindole, a selective delta antagonist. These data suggested that the vagolytic effects of MEAP were elicited via a delta opioid receptor. To test the hypothesis that MEAP’s effects were elicited through a delta opioid receptor mechanism, selective agonists and antagonists for the opioid receptors were utilized. An attenuation of vagal bradycardia was only observed during the infusion of a very selective delta opioid receptor agonist, deltorphin. A mu and kappa agonist showed no significant differences from control. Deltorphin was observed to elicit vagolytic effects in a similar concentration range as MEAP. However, deltorphin was more efficacious that MEAP. There was a significant attenuation of the deltorphin and MEAP’s vagolytic effects, during the co-infusion of the selective delta antagonist, naltrindole. The mu and kappa antagonists were both ineffective. These data further demonstrate that the observed vagolytic effect is linked to a delta opioid receptor. Endogenous MEAP. A series of experiments were undertaken to determine if endogenous MEAP could be demonstrated in the SA node and is so, was it similarly vagolytic. A preconditioning-like protocol was performed to produce intermittent local nodal ischemia to increase the local concentration of endogenous MEAP. The resulting MEAP was measured and was observed to be elevated during the periods of local nodal ischemia and return to control during reperfusion. Contrary to expectations an augmentation of vagal function was observed, during vagal stimulation. The augmented vagal bradycardia was only observed during ischemia, when MEAP was elevated and returned to control during each subsequent reperfusion. Therefore, there was a correlation between elevated MEAP concentrations and augmented vagal bradycardia. The delta antagonist, naltrindole, prevented the augmented vagal response, during nodal ischemia Glibenclamide, a selective KATP channel blocker, partially reversed the augmented vagal response. These data confirm that delta opiate receptors are involved in the augmented vagal bradycardia and that the mechanism may involve the activation of a KATP channel.