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H Høyvik

Publications and source records attributed to H Høyvik.

12 recordsLinked to original sources
PubMed1992

Prelysosomal and lysosomal connections between autophagy and endocytosis.

In isolated rat hepatocytes electroloaded with [14C]sucrose, autophaged sugar accumulated in lysosomes under control conditions, and in prelysosomal autophagic vacuoles (amphisomes) in the presence of asparagine, an inhibitor of autophagic-lysosomal fusion. Endocytic uptake of the sucrose-cleaving enzyme invertase resulted in rapid and complete degradation of autophaged sucrose in both amphisomes and lysosomes. Pre-accumulated sucrose was degraded equally well in both compartments, regardless of amphisomal-lysosomal flux inhibition by asparagine, suggesting that endocytic entry into the autophagic pathway can take place both at the lysosomal and at the amphisomal level. The completeness of sucrose degradation by endocytosed invertase furthermore indicates that all lysosomes involved in autophagy can also engage in endocytosis. Endocytosed invertase reached the amphisomes even when autophagy was blocked by 3-methyladenine, and autophaged sucrose reached this compartment even when endocytic influx was blocked by vinblastine, suggesting that amphisomes may exhibit some degree of permanence independently of either pathway.

Animals↗
PubMed1991

Inhibition of autophagic-lysosomal delivery and autophagic lactolysis by asparagine.

Overall autophagy was measured in isolated hepatocytes as the sequestration and lysosomal hydrolysis of electroinjected [14C]lactose, using HPLC to separate the degradation product [14C]glucose from undegraded lactose. In addition, the sequestration step was measured separately as the transfer from cytosol to sedimentable cell structures of electroinjected [3H]raffinose or endogenous lactate dehydrogenase (LDH; in the presence of leupeptin to inhibit lysosomal proteolysis). Inhibitor effects at postsequestrational steps could be detected as the accumulation of autophaged lactose (which otherwise is degraded intralysosomally), or of LDH in the absence of leupeptin. Asparagine, previously shown to inhibit autophagic but not endocytic protein breakdown, strongly suppressed the autophagic hydrolysis of electroinjected lactose. Vinblastine, which inhibits both types of degradation, likewise suppressed lactose hydrolysis. Asparagine had little or no effect on sequestration, but caused an accumulation of autophaged LDH and lactose, indicating inhibition at a postsequestrational step. Neither asparagine nor vinblastine affected the degradation of intralysosomal lactose preaccumulated in the presence of the reversible lysosome inhibitor propylamine. However, if lactose was preaccumulated in the presence of asparagine, both asparagine and vinblastine suppressed its subsequent degradation. The data thus indicate that autophagic-lysosomal delivery, i.e., the transfer of autophaged material from prelysosomal vacuoles to lysosomes, is inhibited selectively by asparagine and non-selectively by vinblastine.

Animals↗
PubMed1991

Hepatocytic autophagy.

Autophagy is a non-selective bulk process for degradation of cytoplasm, as indicated by ultrastructural evidence and by the similarity in autophagic sequestration rates of various cytosolic enzymes with different half-lifes. The initial autophagic sequestration step is subject to feedback inhibition by amino acids, an effect which is potentiated by insulin and antagonized by glucagon. Epinephrine and other adrenergic agonists inhibit autophagic sequestration through a prazosin-sensitive, alpha 1-adrenergic mechanism. The sequestration is also inhibited by cAMP and by protein phosphorylation as indicated by the effects of cyclic nucleotide analogues, phosphodiesterase inhibitors and okadaic acid. Asparagine specifically inhibits autophagic-lysosomal fusion without having any significant effects on autophagic sequestration, intralysosomal degradation or on the endocytic pathway. Autophaged material that accumulates in prelysosomal vacuoles in the presence of asparagine is accessible to endocytosed enzymes, revealing the existence of an amphifunctional organelle, the amphisome. Evidence from several cell types suggests that endocytosis may be coupled to autophagy in a differential (ligand-dependent) manner, and that amphisomes may play a central role as collecting stations for material destined for lysosomal degradation.

Amino Acids↗
PubMed1989

Energy dependence of different steps in the autophagic-lysosomal pathway.

The energy dependence of the autophagic-lysosomal pathway was investigated in isolated rat hepatocytes, using electroinjected [14C]lactose as an autophagy probe and atractyloside to alter intracellular ATP levels. Since autophagocytosed lactose is hydrolyzed in lysosomes, several steps in the pathway could be analyzed. The following observations were made. 1) The overall autophagic degradation of electroinjected [14C]lactose was strongly energy-dependent. More than 85% inhibition was obtained when the ATP content decreased from the control value of 10 mumol/g dry weight to 4 mumol/g dry weight. 2) The initial step, i.e. the autophagic sequestration of [14C]lactose, measured in the presence of vinblastine to prevent transfer of lactose to lysosomes, was as sensitive to small changes in ATP as was the overall lactose degradation. 3) The steady state level of sequestered [14C]lactose remained constant as ATP decreased from 10 to 4 mumol/g dry weight, indicating that the sequestration step and some postsequestrational process were inhibited to a similar extent by ATP depletion. 4) The final step in the pathway, intralysosomal hydrolysis, was measured by allowing [14C]lactose to preaccumulate intralysosomally in the presence of the reversible lysosome inhibitor propylamine. Following propylamine removal and inhibition of further sequestration by 3-methyladenine, ATP-dependent hydrolysis of the intralysosomal [14C]lactose could be demonstrated. However, this hydrolysis step was not as sensitive to small changes in ATP as was the sequestration step or the overall autophagic lactose degradation. Control of the autophagic-lysosomal pathway in response to energy deprivation would therefore not seem to occur at the lysosomal level, but may be exerted both at the sequestration step and at a postsequestrational, prelysosomal step.

Adenine↗
PubMed1987

Sugar uptake by fluid-phase pinocytosis and diffusion in isolated rat hepatocytes.

Measurements of sugar pinocytosis (fluid-phase endocytosis of radiolabelled sucrose, lactose and raffinose) in freshly isolated rat hepatocytes are disturbed by sugar diffusing into the cells through plasma-membrane blebs. Non-pinocytic entry may be even more pronounced at 0 degrees C, and is a major contributor to 'background' radioactivity. By electrodisruption of the plasma membrane, a distinction can be made between pinocytotically sequestered sugar and free sugar that has entered the cytosol by diffusion. Pinocytosis proceeds at a rate of 2%/h (relative to the intracellular fluid volume), whereas the rate of sucrose entry by diffusion is more than twice as high. Three pinocytotic compartments are distinguishable in isolated hepatocytes: (1) a rapidly recycling compartment, which is completely destroyed by electrodisruption, and which may represent pinocytic channels continuous with the plasma membrane; (2) a non-recycling (or very slowly recycling) electrodisruption-resistant compartment, which allows accumulation of the lysosomally hydrolysable sugar lactose, and which therefore must represent non-lysosomal vacuoles (endosomes?); (3) a lysosomal compartment (non-recycling, electrodisruption-resistant), which accumulates raffinose and sucrose, but which hydrolyses lactose. The last two compartments can be partially resolved in metrizamide/sucrose density gradients by the use of different sugar probes.

Animals↗
PubMed1986

Use of [3H]raffinose as a specific probe of autophagic sequestration.

The trisaccharide [3H]raffinose, introduced into the cytosol of isolated rat hepatocytes by means of electropermeabilization, was sequestered autophagically and accumulated in lysosomes and pre-lysosomal vacuoles. In contrast to the disaccharide [14C]sucrose previously used as a sequestration probe, raffinose was not taken up by the mitochondria. The sequestration of raffinose was completely inhibited by the autophagy suppressor 3-methyladenine, stressing its potential utility as a specific probe of hepatocytic autophagy.

Adenine↗
PubMed1986

Use of a hydrolysable probe, [14C]lactose, to distinguish between pre-lysosomal and lysosomal steps in the autophagic pathway.

[14C]Lactose, introduced into the cytosol of isolated rat hepatocytes by means of electropermeabilization, is sequestered autophagically in the same way as the established sequestration probe, [14C]sucrose. However, unlike the inert sucrose molecule, lactose is rapidly hydrolysed in the lysosomes, and can therefore be used to probe the last step of the autophagic pathway (i.e. fusion with the lysosome). During autophagy lactose is present only at a low, steady-state level in pre-lysosomal vacuoles (probably autophagosomes), serving as a useful marker for these organelles. If autophagosome-lysosome fusion is blocked with vinblastine (Kovács et al., Exp cell res 137 (1982) 191), [14C]lactose will accumulate continuously as a function of the sequestration rate, and reach a high level in the pre-lysosomal vacuoles. Density gradient analysis, using chloroquine (CLQ) to alter lysosomal density, suggests that these organelles have a broad density distribution (1.08-1.13 g/ml), thus differing significantly from the distribution of lysosomes.

Adenine↗
PubMed1986

Radiolabelled sugars as probes of hepatocytic autophagy.

The sugars [14C]sucrose, [14C]lactose and [3H]raffinose have been loaded into isolated rat hepatocytes by means of electropermeabilisation, and used as probes of autophagic-lysosomal function. All three sugars are sequestered autophagically; in addition sucrose and lactose are taken up by mitochondria, and lactose is hydrolysed intralysosomally. Autophagically sequestered radioactivity accumulates in relatively large vacuoles (not in microsomes). These vacuoles initially have a density which, on average, is lower than that of the lysosomes. With time, the lysosomes become lighter and coincident with the radioactive vacuoles in density gradients, a phenomenon that is partially prevented by the autophagy inhibitor 3-methyladenine (3MA). This may indicate that resting, dense lysosomes become active (light) by engaging in autophagy as well as in other processes (endocytosis?). To assess the extent of convergence of the autophagic and endocytic pathways, the effect of endocytosed invertase on autophagically sequestered sucrose has been investigated. Practically all of the sucrose is eventually degraded the enzyme, indicating a complete intermixing of autophagic and endocytic pathways at the level of the lysosome. The density of lysosomes can be increased by treatment of cells with leupeptin. Most of the vaculoes containing sequestered radioactivity also increase their density under these conditions, even in the presence of vinblastine, an inhibitor of autophagosome-lysosome fusion. This indicates that a substantial fraction of the autophagosomes may be physically associated with lysosomes prior to fusion, a hypothesis supported by electron microscopic observations. Both autophagic sequestration (of raffinose) and autophagic-lysosomal degradation (of lactose, measured by an HPLC method) is completely inhibited by 3MA, whereas endocytic-lysosomal degradation (of asialofetuin) is unaffected. Since endogenous protein degradation is only partially (70%) suppressed by 3MA, it is clear that the remaining degradation must be non-autophagic (non-sequestrational), and most likely non-lysosomal.

Animals↗
PubMed1979

Binaural hearing spectacles with "no mould" by acoustic trauma.

Patients with selective hearing loss in the trebles, as commonly seen in acoustic trauma, often have a considerable hearing handicap. They seldom benefit from the usual hearing aids with closed ear moulds. When Ewertsen (1970) and Cortois (1972) reported their experiences with binaural fitting of hearing spectacles, using a free plastic tube in the ear canal ("no mould"), we at the Hearing Centre in Namsos began to fit such hearing aids to 33 patients with acoustic traumas. All of them were followed-up in February--March 1978. The results have been evaluated on the basis of answers from the patients entered on questionnaires, and speech audiometry in open field, monosyllable, with 60 dB wide-band background noise. 27 patients (80%) stated that they were greatly satisfied, and 7 (20%) well satisfied. The average daily use was 4 1/2 hrs. All of the patients emphasized the great benefit obtained with the hearing aid at conferences, meetings, social occasions, etc. The average hearing gain was 13 dB. Among those who had discrimination loss there was an average gain of 30% .

Adult↗