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M C Andresen

Publications and source records attributed to M C Andresen.

48 records · Page 3Linked to original sources
PubMed1982

Effects of changes in extracellular ionic concentrations on aortic baroreceptors with nonmyelinated afferent fibers.

The effects of ionic change upon one of the two classes of aortic baroreceptors, those having unmyelinated axons or C-fibers, have not been examined heretofore. Differences from results in aortic baroreceptors with myelinated axons might be expected because of differences in accessible surface area-volume relationships. Recordings were obtained using an in vitro aortic arch-aortic nerve preparation from 21 aortic C-fibers in 15 normotensive Wistar-Kyoto rats (WKR) 13-17 weeks old. During a slow pressure ramp, the endings responded with an irregular discharge. However, if the discharge was averaged over 10-20 seconds, the average discharge correlated well with the pressure level in the arch. The maximal discharge increased with faster ramp rates but the threshold was not shifted over the range of ramp rates employed. The relationship between pressure and discharge was tested during alterations in the extracellular concentrations of Na, K, and Ca ([Na]0, [K]0, and [Ca]0). A 12% decrease in [Na]0 clearly decreased the firing in the aortic C-fibers during a ramp stimulus. During a pressure step, the initial discharge was not decreased but the steady state discharge was clearly depressed. When the aortic arch was perfused continuously at constant pressure, a decrease in Na of only 6% depressed the activity. Doubling [K]0 increased firing for a few minutes and was followed by a reduction of discharge. Previous reports have shown no effect on myelinated baroreceptor discharge by a 12% decrease in [Na]0, a change which significantly depressed firing in all C-fiber baroreceptors in this study. Thus, C-fiber baroreceptors are more sensitive to changes in extracellular ionic composition than are baroreceptors with myelinated axons.

Afferent Pathways↗
PubMed1980

Baroreceptor function in spontaneously hypertensive rats. Effect of preventing hypertension.

In the previous paper we showed that baroreceptor resetting in spontaneously hypertensive rats (SHR) resulted from an incomplete matching of increased receptor strain sensitivity with reduced vessel wall distensibility. Here SHR and Wister-Kyoto controls were treated with antihypertensive drugs in their drinking water (reserpine, hydrochlorothiazide, and hydralazine) from 5 weeks of age so that we might examine the role of blood pressure in the SHR vessel wall-receptor mismatch. Radius and receptor properties were measured using the in vitro aortic arch-aortic nerve preparation at 8, 14, 20, and 30 weeks. We found that baroreceptors from SHR with normal blood pressures had normal pressure thresholds and suprathreshold pressure sensitivities. However, the treated SHR receptors still had lower strain thresholds and the aortas still were less distensible than were the normal. Now, however, the increased receptor sensitivity was sufficient to normalize the relationship between baroreceptor function and arterial blood pressure. We conclude that, although blood pressure may be restored to normotensive levels by therapy, vascular abnormalities may continue to develop.

Aging↗
PubMed1979

Ultrastructure of Aplysia neurons having different degrees of light sensitivity.

The relationship between ultrastructure and photosensitivity of pigmented neurons of the abdominal ganglion of Aplysia californica was investigated using electron microscopy and electrophysiological methods. Four identified neurons of similar light microscopic appearance were examined; two are photoresponsive and two are not. Illumination hyperpolarizes both responsive neurons. One of them, R2, requires roughly 100 times greater light intensities than does the other, the ventral photoresponsive neuron (VPN), for similar responses. Two neurons lying adjacent to VPN and similar in appearance to VPN do not have measurable electrophysiological responses to even the highest light intensities. All four neurons contained lipochondria, pigmented organelles associated with the light response. Therefore the presence of these organelles is not the only requirement for light sensitivity in these neurons. Illumination appeared to increase the number of membranous lipochondria in both R2 and the ventral neurons, but only in R2 was this increase significant. Factors such as the concentration of lipochondria near the plasma membrane may affect quantitative aspects of the light response, but in the insensitive cells the lipochondria are apparently uncoupled from other factors required for the light response.

Animals↗
PubMed1979

Photoresponses of a sensitive extraretinal photoreceptor in Aplysia.

1. The light-evoked membrane current, photo-current, of an extraretinal photo-receptor, the ventral photoresponsive neurone (v.p.n.), in the abdominal ganglion of Aplysia californica, was studied using the voltage clamp method. Flashes and steps of monochromatic light were used as stimuli. 2. Flashes of light 100 msec in duration elicit slowly developing outward currents which peak at 5--10 sec and then return to dark levels within 30--60 sec. 3. The peak of the action spectrum of v.p.n. is at 470 nm and is similar to the peak for R2, another photoresponsive extraretinal Aplysia neurone, and to the peak of absorption spectra of molluscan rhodopsins. V.p.n. also contains membrane-bound cytoplasmic pigmented granules similar to those found in R2, and these are thought to mediate the light response. 4. Photo-current is associated with an increase in membrane conductance. In normal sea water photo-current has a reversal potential at the K equilibrium potential, EK and the reversal potential has a Nernstian relationship with external K concentration. The current--voltage relationships for peak and steady-state photo-current are fitted by the same constant field equation; currents measured when voltage was changed in steps at peak photo-current also have a similar relationship with voltage. The results are similar when saturating or non-saturating light intensities were used. Thus it appears that the light-activated K+ conductance is neither time nor voltage dependent. 5. Minimally detectable responses occurred at flash photon densities of 10(12) photons cm-2 which is 10(-3) that for R2. This value is comparable to those reported for retinal photoreceptors of Pecten irradians, a scallop, and Salpa democratica, a pelagic tunicate, and is lower than values reported for extraretinal photoreceptors such as the pineal photoreceptors of Salmo gairdnerii irideus, the rainbow trout, and the caudal photoreceptor in the sixth abdominal ganglion of Procambarus clarkii, a crayfish. 6. V.p.n. has a linear amplitude response range for low intensities of light and a non-linear range that saturates at high intensities. In the accompanying paper the response wave form and its temperature dependence are interpreted according to a diffusion-based model.

Animals↗
PubMed1979

The role of diffusion in the photoresponse of an extraretinal photoreceptor of Aplysia.

1. Membrane currents produced by flashes and steps of light (photo-current) were recorded from the ventral photoresponsive neurone of Aplysia californica. The effects of background illumination and changes in temperature were also examined. 2. The falling phase of the response wave form may be separated into two components with time constants of 10--12 sec and 50 sec. 3. Background illumination reduced the response amplitude to light impulses without appreciably altering the response wave form. 4. Lowering the temperature greatly reduced the amplitude of the photo-current with a Q10 of 2.91 (25--15 degrees C) and greatly prolonged the duration of the response. 5. Because of the relatively large distance between the plasma membrane and the pigmented cytoplasmic lipochondria where light is absorbed, a diffusion-based model with Ca as the internal-transmitter (Andresen & Brown, 1979) was developed. 6. In this model diffusion of Ca2+ released from the lipochondria upon photon absorption is slowed by the reversible uptake of Ca2+ at cytoplasmic binding sites. Ca2+ interacts with sites at the plasma membrane to increase GK and Ca2+ levels are subsequently restored by irreversible uptake processes. Ca2+ release and its adsorption and desorption from the more numerous plasma membrane binding sites were assumed to be instantaneous with respect to the duration of the light-evoked response. 7. The linearized model equations adequately predict the experimental response wave forms, the effects of temperature, and saturation of the steady-state amplitude--stimulus relationship. Aside from amplitude scaling, no curve-fitting was used. 8. The model also gives realistic values for the cytoplasmic diffusion coefficient of Ca and the net rate of Ca efflux required to restore dark Ca activity.

Animals↗
PubMed1978

Relationship of aortic wall and baroreceptor properties during development in normotensive and spontaneously hypertensive rats.

We studied the relationship between aortic baroreceptor function and aortic wall properties in normotensive (NTR) and spontaneously hypertensive (SHR) rats 10-20 weeks old. Baroreceptor discharge, static pressure-volume (P-V), and pressure-radius relationships were measured in excised aortic segments. Histological studies of wall thickness and receptor numbers also were made. Circumferential wall stress and strain were calculated, as was the incremental elastic modulus (EINC). EINC in NTR's at 100 mm Hg was similar to values reported for in vivo human, dog, and rat aortas. At 10 weeks, SHR's had significantly elevated blood pressure, but SHR and NTR aortas had similar relationships among pressures, volumes, strains, and EINC's. Differences arose subsequently and, at 20 weeks, NTR aortas had larger volumes, larger strains, and smaller EINC's at equivalent pressures, whereas SHR aortas were unchanged. Thus the reduced distensibility of SHR relative to NTR aortas, rather than being due to retrogressive changes from normal, appeared to result from a failure to pass through a phase of increased distensibility. At 10 weeks, SHR baroreceptors showed resetting in both pressure-response and strain-response curves, and it was concluded that early hypertensive baroreceptor resetting was due to primary changes in the receptors. At 20 weeks, the order of the strain-response curves for NTR and SHR baroreceptors was reversed due to a reduction in strain sensitivity of NTR baroreceptors. Resetting of NTR baroreceptors during development may have important implications as a mechanism of blood pressure control in development.

Animals↗