Monovalent currents through the T-type Cav3.1 channels and their block by Mg2+.

We have investigated the permeability of the Cav3.1 channel for Ca2+ and different monovalent cations and the block of the currents by Mg2+ ions. In the absence of extracellular divalent cations, the Cav3.1 channel was more permeable for Na+ than for Cs+ and impermeable for NMDG+. Monovalent currents were inhibited by Mg2+ of near physiological concentration by three orders of magnitude more effectively than the Ca2+ current. Inhibition of outward, but not inward current by Mg2+ was voltage-dependent. Furthermore, magnesium slowed down channel deactivation presumably by interacting with an open channel state.

Modulation by 6-hydroxydopamine of expression of the phenylethanolamine N-methyltransferase (PNMT) gene in the rat heart during immobilization stress.

Phenylethanolamine N-methyltransferase (PNMT) is the final enzyme in the catecholamine synthesizing cascade that converts noradrenaline (NA) to adrenaline (Adr). Both of these catecholamines are physiologically important hormones and neurotransmitters in mammals with profound influence on the activity of the cardiovascular system. Although PNMT activity and gene expression have been reported in the neonatal and also adult rat heart, little is known about the identity of the cells expressing PNMT mRNA. In this study, we have shown that besides PNMT in neuronal and intrinsic cardiac cells, this enzyme is expressed also in rat cardiomyocytes, as shown by immunofluorescence in isolated cardiomyocytes. To determine which cells in the heart more sensitively show stress-induced changes in PNMT mRNA expression, we performed chemical sympathectomy by administration of 6-hydroxydopamine (6-OHDA), which destroys catecholaminergic terminals. We determined PNMT mRNA levels in the left atria and ventricles of control and stressed rats. In the rats treated with 6-OHDA, PNMT mRNA levels were not changed under normal, physiological conditions compared to vehicle treated rats. Similar results were observed on isolated cardiomyocytes from control and 6-OHDA treated rats. However, 6-OHDA treatment prevented immobilization-induced increase in PNMT mRNA expression. The results allow us to propose that in the heart, the immobilization-induced increase in PNMT gene expression is probably not in cardiomyocytes, but in neuronal cells.

Structure and composition of tubular aggregates of skeletal muscle fibres.

Unusual regions of densely packed membranous tubules known as tubular aggregates (TAs) have been observed in skeletal muscle fibres of mammals under numerous pathological conditions but also in health. Their causality is unclear. It is neither known whether TAs are destructive and should be treated or whether they have a compensating function in an endangered muscle. In spite of many similarities, the histochemical, immunocytochemical and ultrastructural characteristics of tubular aggregates do vary. Histochemistry provided an overall characteristic of TAs as membranous inclusions with a variety of enzymatic activities. Immunocytochemical evidence revealed that tubular aggregates contain miscellaneous proteins and that derive from membranes of sarcoplasmic reticulum and mitochondria. No evidence for the presence of contractile and cytoskeletal proteins in TAs was found. Ultrastructurally, TAs are characterized as more or less densely packed aggregates of vesicular or tubular membranes of variable forms and sizes that may contain amorphous material, filaments or inner tubules. Various reported types of tubular aggregates, namely, proliferating terminal cisterns, vesicular membrane collections, TAs with double-walled tubules, TAs with single-walled tubules, aggregates of dilated tubules with inner tubules, aggregates of tubulo-filamentous structures, filamentous tubules, riesentubuli, and related membranous structures including cylindrical spirals are sumarized and analyzed here in detail.