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Biomedical subjects

R D Shonat

Publications and source records attributed to R D Shonat.

9 recordsLinked to original sources
PubMed1999

Critical PO(2) of skeletal muscle in vivo.

The main purpose of this study was to determine the interstitial oxygen tension at which aerobic metabolism becomes limited (critical PO(2)) in vivo in resting skeletal muscle. Using an intravital microscope system, we determined the interstitial oxygen tension at 20-micrometer-diameter tissue sites in rat spinotrapezius muscle from the phosphorescence lifetime decay of a metalloporphyrin probe during a 1-min stoppage of muscle blood flow. In paired experiments NADH fluorescence was measured at the same sites during flow stoppage. NADH fluorescence rose significantly above control when interstitial PO(2) fell to 2.9 +/- 0.5 mmHg (n = 13) and was not significantly different (2.4 +/- 0.5 mmHg) when the two variables were first averaged for all sites and then compared. Similar values were obtained using the abrupt change in rate of PO(2) decline as the criterion for critical PO(2). With a similar protocol, we determined that NADH rose significantly at a tissue site centered 30 micrometer from a collecting venule when intravascular PO(2) fell to 7.2 +/- 1.5 mmHg. The values for critical interstitial and critical intravascular PO(2) are well below those reported during free blood flow in this and in other muscle preparations, suggesting that oxygen delivery is regulated at levels well above the minimum required for oxidative metabolism. The extracellular critical PO(2) found in this study is slightly greater than previously found in vitro, possibly due to differing local conditions rather than a difference in metabolic set point for the mitochondria.

Animals↗
PubMed1997

Near-simultaneous hemoglobin saturation and oxygen tension maps in mouse brain using an AOTF microscope.

A newly developed microscope using acousto-optic tunable filters (AOTFs) was used to generate in vivo hemoglobin saturation (SO2) and oxygen tension (PO2) maps in the cerebral cortex of mice. SO2 maps were generated from the spectral analysis of reflected absorbance images collected at different wavelengths, and PO2 maps were generated from the phosphorescence lifetimes of an injected palladium-porphyrin compound using a frequency-domain measurement. As the inspiratory O2 was stepped from hypoxia (10% O2), through normoxia (21% O2), to hyperoxia (60% O2), measured SO2 and PO2 levels rose accordingly and predictably throughout. A plot of SO2 versus PO2 in different arterial and venous regions of the pial vessels conformed to the sigmoidal shape of the oxygen-hemoglobin dissociation curve, providing further validation of the two mapping procedures. The study demonstrates the versatility of the AOTF microscope for in vivo physiologic investigation, allowing for the generation of nearly simultaneous SO2 and PO2 maps in the cerebral cortex, and the frequency-domain detection of phosphorescence lifetimes. This class of study opens up exciting new possibilities for investigating the dynamics of hemoglobin and O2 binding during functional activation of neuronal tissues.

Acoustics↗
PubMed1997

Oxygen tension gradients and heterogeneity in venous microcirculation: a phosphorescence quenching study.

Localized measurements of intravascular oxygen tension (PO2) at multiple locations in the microvascular network of the rat spinotrapezius muscle were used to study the spatial distribution of PO2 in venular structures. By use of a newly developed phosphorescence system to rapidly and repeatedly measure PO2, 538 individual measurements were made in 18 different networks during rest. Average intravascular PO2 was (in mmHg +/- SD) 33 +/- 9, 21 +/- 9, 26 +/- 10, and 33 +/- 8 in small arcade arterioles, postcapillary venules (PV), 3 degrees venules (3V), and arcade venules, respectively. The coefficient of variation (CV), a descriptive indicator of spatial heterogeneity, was correspondingly 0.28, 0.45, 0.37, and 0.23 for the different vessel groups. PO2 was found to increase significantly (P < 0.001) from PV to 3V, rising 0.009 +/- 0.002 mmHg/microns along the vessel. By linear regression, the slope of PO2 for the vessel difference group, PV-3V as a function of mean systemic blood pressure (BPm; in mmHg) was -0.09 +/- 0.04 (P < 0.05), indicating that the measured longitudinal oxygen gradients and CV are only weakly dependent on BPm. The results support the hypothesis that oxygen can diffuse across the walls of postcapillary vessels and suggest that the venular structures are not merely passive conduits for removing oxygen and waste products but may play an important role in regulating oxygen delivery.

Animals↗
PubMed1993

O2 gradients and countercurrent exchange in the cat vitreous humor near retinal arterioles and venules.

Recessed cathode O2 microelectrodes were used to measure spatially detailed oxygen tension (PO2) gradients in the vitreous humor near the cat retina. Measurement sites (n = 41 in 8 cats) included single arterioles and venules and parallel vessel pairs. Mean vitreous PO2 was 37.9 +/- 1.5 (SE) Torr. Close to the retinal surface (approximately 200 microns), PO2 was found to be both higher and lower than the vitreous PO2, depending on the proximity of the microelectrode tip to retinal vessels. Both positive (inward) and negative (outward) O2 fluxes (JO2) were measured, consistent with the anatomy and expected boundary conditions in the eye. The PO2 at the closest approach above arterioles was 55.2 +/- 2.3 Torr, significantly higher than in the vitreous (P < 0.0001). All arterioles had outward JO2 with an overall mean of -2.58.10(-6) ml O2/sec/cm2. Some of the venules were also losing O2, but at much lower rates than arterioles. Several venules were gaining O2. Countercurrent transport (A-V shunting) was also seen between vessel pairs. Our experimental results allow theoretical predictions to be made for the axial drop in blood PO2 along an arteriole as a function of blood flow.

Animals↗
PubMed1992

Laser Doppler flowmetry in the optic nerve.

Laser Doppler flowmetry (LDF) is a technique that measures relative average velocity, number and flux (number times velocity) of red blood cells in a tissue. In this paper, we demonstrate its application in the optic nerve head tissue, describe the laser delivery and light scattering detection schemes and investigate the effect of the distance between the sites of illumination and detection. We also provide evidence that the flow measured by LDF varies linearly with actual blood flow in the optic nerve and examine the question of the depth of the sampled volume. Experiments in anesthetized cats illustrate potential applications which make use of the high temporal resolution of LDF. These include the response of blood flow to changes in the composition of the breathing gases and changes induced by neuronal stimulation with multiple and single flashes.

Animals↗
PubMed1992

Effect of acute increases in intraocular pressure on intravascular optic nerve head oxygen tension in cats.

A newly developed phosphorescence imaging technique was used to generate two-dimensional maps of intravascular oxygen tension (PO2) in the optic nerve head (ONH) and retina of the cat to study the effects of acute moderate increases in intraocular pressure (IOP) on the ONH and retinal PO2. Both the ONH and retinal PO2 were remarkably well maintained as the IOP increased; hypoxia developed only after the blood flow to the eye was stopped. Because ONH hypoxia was not observed during IOP elevation, a lack of oxygen may not be a major cause of glaucomatous damage, although the effects of chronically elevated IOP on the PO2 remain to be evaluated. Because this imaging technique was noninvasive and required only a small bolus injection of a nontoxic oxygen probe, the authors anticipate that it will find significant application in the study of many ocular vascular diseases and glaucoma.

Acute Disease↗
PubMed1991

Flicker evoked increase in optic nerve head blood flow in anesthetized cats.

The effect of diffuse luminance flicker stimulation of a large area (approximately 30 degrees diameter) on red blood cell flux (F) in the optic nerve head was measured in the anesthetized cat. F increased markedly during sustained flicker. The F-response to the initiation and cessation of the stimulation was found to occur within a few seconds. Upon sustained stimulation, the increase in F reached a plateau within approximately 2 min. Its level depended upon the intensity, frequency and wavelength of the stimulation and the state of adaptation of the retina. This stimulus offers a new and powerful means of investigating blood flow regulation in the optic nerve head (ONH).

Adaptation, Ocular↗
PubMed1991

New ocular micromanipulator for measurements of retinal and vitreous physiologic parameters in the mammalian eye.

A new ocular micromanipulator for introducing a microelectrode into the mammalian eye is described. Included is a technical description of the device, incorporating arched tracks and DC motors to provide for angular motion around a scleral entrance point. Applications for in vivo measurements of local PO2 in the retina and vitreous of miniature pigs and cats are shown. The new design features of this micromanipulator allow for smooth microelectrode movements over large retinal areas, facilitating the rapid collection of data in many different areas of the retina and vitreous.

Animals↗