Access Type

Open Access Dissertation

Date of Award

January 2013

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physiology

First Advisor

Tadeusz J. Scislo

Abstract

ABSTRACT

by

ZELJKA MINIC

August 2013

Advisor: Dr. Tadeusz J. Scislo

Major: Physiology

Degree: Doctor of Philosophy

Adenosine is an important neuromodulator of cardiovascular control at the level of the nucleus of the solitary tract (NTS) where cardiovascular and other autonomic reflexes are primarily integrated. Levels of adenosine increase in the NTS during life threatening hypotension, ischemia and hypoxia. Adenosine may modulate cardiovascular reflexes to correct hemodynamic imbalance, acting via A1 and A2a receptors which inhibit and facilitate neurotransmitter release, respectively. Previous studies from our laboratory showed that NTS A1 adenosine receptors inhibit the arterial baroreflex whereas A2a receptors do not reset baroreflex control of regional sympathetic outputs, although stimulation of these receptors decreases renal (RSNA) increases preganglionic adrenal (pre-ASNA) and does not alter lumbar (LSNA) sympathetic nerve activity. Cardiopulmonary chemoreflex (CCR), also known as Bezold-Jarisch reflex, triggered by various chemicals inhaled or released during myocardial ischemia, may have beneficial effects when it decreases afterload and heart rate thus decreasing oxygen demand. However, strong activation of the CCR may cause severe bradycardia and hypotension leading to hypoperfusion of vital organs, fainting and even death. During this severe reflex hypotension adenosine may be release into NTS. Then adenosine may inhibit the reflex to regain hemodynamic balance. A1 adenosine receptors likely inhibit glutamatergic transmission the CCR pathway as they inhibit the arterial baroreflex mediated via similar medullary pathways and transmitters. Interestingly, preliminary data showed that NTS A2a facilitatory receptors also inhibit the CCR. Therefore I hypothesized that adenosine operating via both receptor subtypes may act as negative feedback regulator for CCR control of regional sympathetic outputs acting via two different mechanisms: A1 receptors mediated direct inhibition and A2a receptor mediated facilitation of release of an inhibitory neurotransmitter (most likely GABA) which in turn inhibits the CCR pathway. Since both NTS A1 and A2a adenosine receptors exert differential effects on regional sympathetic outputs I anticipated that the inhibition of CCR control of regional sympathetic outputs will be also regionally specific.

In urethane-chloralose-anesthetized rats I compared regional reflex sympathoinhibition evoked by activation of cardiopulmonary receptors with intra-atrial injections of phenylbiguanide (1-8 microg/kg) before and after stimulation of NTS A1 and A2a adenosine receptors via microinjections into the NTS of N6cyclopentyl adenosine (0.33-330 pmol/50 nl) and CGS 21680 (0.2-20 pmol/50 nl), respectively. The comparisons were made also following nonselective blockade of A1 and A2a receptors ((8-(p-sulfofenyl) theophyline, 1 nmol/100 nl)) and selective blockade of A2a receptors (ZM-2143285, 40 pmol/ 100 nl) and following respective volume controls. Gradual stimulation of both A1 and A2a adenosine receptors located in the NTS caused similar dose dependent inhibition of hemodynamic and regional sympathetic responses. The inhibition was independent of differential effects of adenosine receptor subtypes in regional sympathetic activity suggesting that these two effects are mediated via different mechanisms in the NTS. Blockade of adenosine receptor subtypes did not affect the CCR showing that under normal conditions adenosine levels are too low to affect the reflex. Blockade of GABAA receptors in the NTS (Bicuculline, 10 pmol/100 nl) removed A2a receptor mediated inhibition of the reflex whereas blockade of GABAB receptors (SCH-50911, 1 nmol/100 nl) attenuated the A2a receptor mediated inhibition to a lesser extent. I found also that GABA operating via GABAA but not GABAB receptors exerts small, but significant tonic inhibition of the CCR. In addition, simultaneous recordings from the three sympathetic outputs showed that CCR differentially inhibits regional sympathetic outputs (RSNA>pre-ASNA>LSNA); this observation clarified inconsistent reports on CCR control of these sympathetic outputs.

I conclude that adenosine may act as a negative feedback regulator of the CCR control of neural and hemodynamic variables. NTS A1 receptors directly inhibit glutamatergic transmission in the reflex pathway, whereas A2a receptors facilitate release of GABA which inhibits the CCR pathway mostly via GABAA receptors.

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