PubMed Health⌕ Search

Biomedical subjects

R Millecchia

Publications and source records attributed to R Millecchia.

7 recordsLinked to original sources
PubMed1984

Cellular potentials, electrogenic sodium pumping and sensitivity in guinea-pig atria.

Intracellular recording techniques in guinea-pig atrial pacemaker and nonpacemaker cells were used to investigate 1) the role of membrane potential changes in postjunctional supersensitivity, 2) the electrogenicity of the Na+,K+ pump and 3) the role of electrogenic pumping in sensitivity of the atria to agonists. In nonpacemaker cells, ouabain (10(-6) M) had no effect on resting membrane potential (left atria) or maximum diastolic potential (right atria). However, ouabain effectively suppressed the transient hyperpolarization that followed cessation of electrical stimulation. In pacemaker cells, ouabain and chronic treatment with reserpine (0.1 mg/kg/day) produced quite different patterns of changes in intracellular potentials. Chronic treatment with reserpine induced chronotropic supersensitivity to isoproterenol but not to histamine. Ouabain did not alter the chronotropic sensitivity to either agonist. The effects of isoproterenol and histamine on intracellular potentials in pacemaker cells were investigated in the presence and absence of ouabain and in control atria vs. atria from guinea pigs chronically pretreated with reserpine. Analysis of the data indicated that 1) electrophysiological measurements do not provide a discernible explanation for chronotropic supersensitivity, 2) the Na+ pump has the capacity for electrogenic pumping under conditions of Na+ loading, but demonstrates little indication of electrogenicity under basal conditions and 3) chronic treatment with reserpine does suppress the Na+,K+ pump in some areas of the right atrium, but this activity probably does not contribute to chronotropic supersensitivity. Other possible mechanisms of postjunctional supersensitivity in atria are discussed.

Animals↗
PubMed1972

Photoreception in a barnacle: electrophysiology of the shadow reflex pathway in Balanus cariosus.

The photoreceptors in the median ocellus of the rock barnacle depolarize when illuminated. This depolarization spreads passively to the axon terminals in the supraesophageal ganglion. A small number of cells in the supraesophageal ganglion hyperpolarize when the median ocellus is illuminated and depolarize when it is shadowed. Nerve impulses are superimposed on the slow depolarization of the ganglion cells. Impulse activity in response to shadowing the median ocellus is recorded in a few fibers of the circumesophageal connectives. Picrotoxin blocks this shadow-induced activity. A model of the shadow reflex pathway is presented.

Animals↗
PubMed1969

The ventral photoreceptor cells of Limulus. I. The microanatomy.

The ventral photoreceptor cells of Limulus polyphemus resemble the retinular cells of the lateral eyes both in electrical behavior and in morphology. Because of the great size of the ventral photoreceptor cells they are easy to impale with glass capillary micropipettes. Their location along the length of the ventral eye nerve makes them easy to dissect out and fix for electron microscopy. Each cell has a large, ellipsoidal soma that tapers into an axon whose length depends upon the distance of the cell from the brain. The cell body contains a rich variety of cytoplasmic organelles with an especially abundant endoplasmic reticulum. The most prominent structural feature is the microvillous rhabdomere, a highly modified infolding of the plasmalemma. The microvilli are tightly packed together within the rhabdomere, and quintuple-layered junctions are encountered wherever microvillar membranes touch each other. Glial cells cover the surface of the photoreceptor cell and send long, sheet-like projections of their cytoplasm into the cell body of the photoreceptor cell. Some of these projections penetrate the rhabdomere deep within the cell and form quintuple-layered junctions with the microvilli. Junctions between glial cells and the photoreceptor cell and between adjacent glial cells are rarely encountered elsewhere, indicating that there is an open pathway between the intermicrovillous space and the extracellular medium. The axon has a normal morphology but it is electrically inexcitable.

Animals↗
PubMed1969

The ventral photoreceptor cells of Limulus. II. The basic photoresponse.

The ventral photoreceptors of Limulus polyphemus are unipolar cells with large, ellipsoidal somas located long both "lateral olfactory nerves." As a consequence of their size and location, the cells are easily impaled with microelectrodes. The cells have an average resting potential of -48 mv. The resting potential is a function of the external concentration of K. When the cell is illuminated, it gives rise to the typical "receptor potential" seen in most invertebrate photoreceptors which consists of a transient phase followed by a maintained phase of depolarization. The amplitude of the transient phase depends on both the state of adaptation of the cell and the intensity of the illumination, while the amplitude of the maintained phase depends only on the intensity of the illumination. The over-all size of the receptor potential depends on the external concentration of Na, e.g. in sodium-free seawater the receptor potential is markedly reduced, but not abolished. On the other hand lowering the Ca concentration produces a marked enhancement of both components of the response, but predominantly of the steady-state component. Slow potential fluctuations are seen in the dark-adapted cell when it is illuminated with a low intensity light. A spike-like regenerative process can be evoked by either the receptor potential or a current applied via a microelectrode. No evidence of impulse activity has been found in the axons of these cells. The ventral photoreceptor cell has many properties in common with a variety of retinular cells and therefore should serve as a convenient model of the primary receptor cell in many invertebrate eyes.

Animals↗
PubMed1969

The ventral photoreceptor cells of Limulus. 3. A voltage-clamp study.

In the dark, the ventral photoreceptor of Limulus exhibits time-variant currents under voltage-clamp conditions; that is, if the membrane potential of the cell is clamped to a depolarized value there is an initial large outward current which slowly declines to a steady level. The current-voltage relation of the cell in the dark is nonlinear. The only ion tested which has any effect on the current-voltage relation is potassium; high potassium shifts the reversal potential towards zero and introduces a negative slope-conductance region. When the cell is illuminated under voltage-clamp conditions, an additional current, the light-induced current, flows across the cell membrane. The time course of this current mimics the time course of the light response (receptor potential) in the unclamped cell; namely, an initial transient phase is followed by a steady-state phase. The amplitude of the peak transient current can be as large as 60 times the amplitude of the steady-state current, while in the unclamped cell the amplitude of the peak transient voltage never exceeds 4 times the amplitude of the steady-state voltage. The current-voltage relations of the additional light-induced current obtained for different instants of time are also nonlinear, but differ from the current-voltage relations of the dark current. The ions tested which have the greatest effect on the light-induced current are sodium and calcium; low sodium decreases the current, while low calcium increases the current. The data strongly support the hypothesis that two systems of electric current exist in the membrane. Thus the total ionic current which flows in the membrane is accounted for as the sum of a dark current and a light-induced current.

Animals↗
PubMed1966

Simple photoreceptors in Limulus polyphemus.

The "olfactory nerve," the endoparietal eye, and the rudimentary lateral eyes of Limulus (polyphemus) contain simple photoreceptor cells that duplicate many of the electrical responses of the retinular cells of the lateral eye; the responses are a receptor potential consisting of aninitial transient phase and a subsequent steady phase,low-amplitude fluctuations, and a small locally regenerative response to pulses of both light and current. Photic stimulation does not induce conducted action potentials, but does increase the membrane conductance. The receptor potentialrequires the presence of sodium ions in the external medium. Measurements of action and absorption spectra indicate a photopigment whose maximum absorption is of light with wavelength of 535 nanometers. The functional significance of these cells has not been ascertained.

Animals↗