Stefan B. Sigurdsson

Strontium and Barium as Substitutes for Calcium on Electrical and Mechanical Activity in Rat Portal Vein

Sr (above 0.5 mm) and Ba (above 0.25 mm) can maintain spontaneous electrical activity of portal vein in the absence of Ca. This is true also in Na-free solution. An approximately normal pattern of activity with well-defined periods of quiescence is obtained with Sr at lower concentrations than with Ca. Ba causes continuous tetanic spike discharge. Excitation-contraction coupling appears less effective with Sr and Ba than with Ca. These differences can explain the different concentration-effect curves obtained for the three ions with regard to integrated isometric force. The contracture responses of depolarized portal vein induced by Ca, Sr or Ba decreased by 27, 44 and 63 %, respectively, after 30 min in Ca-free solution with 1 mm EGTA. No further decrease occurred after 2 h. The mechanical responses to Sr and Ba appear to be combined effects of liberation of tissue Ca and direct action on the contractile system.

The effect of hypoxia on mechanical and electrical properties of smooth muscle from the rat portal vein

The effect of hypoxia on the electrical and mechanical activity of rat portal vein smooth muscle were investigated using intracellular microelectrode technique in combination with contraction force measurements. In control conditions the “resting” potential of the muscle cells was −58.0 ±0.1 mV (mean±S.E.). Bursts of action potentials, 5–10s long, appeared at regular intervals (2–3/min) in association with phasic contractions. In hypoxia (PO2, at about 10 mm Hg) there was a marked decrease in force and, often, a moderate increase in rate of the spontaneous contractions. Electrically, these changes corresponded to a decrease in length and an increase in frequency of the bursts of action potentials. The general level of membrane polarization and individual action potentials were not affected. In prolonged hypoxia there was a tendency towards dissociation of the electrical and mechanical activities. Increasing [K+]0 or reducing [Na+]0 produced a restoration of the hypoxically depressed force development. Qualitatively similar resuts with respect to the depression of myogenic activity by hypoxia, and its alleviation in decreased [K+]0 and reduced [Na+]0, were obtained in low Ca (1 mM) solutions. The effects of hypoxia, and their variation with [K+]0 and [Na+]0, could be explained on the basis of pH dependent electro-mechanical uncoupling.

Effects of Strontium and Some Other Divalent Cations on Electrical and Mechanical Activity in Rat’s Portal Vein

Smooth muscle cells as well as striated muscle cells contain the proteins actin and myosin, which constitute the thin and thick myofilaments, respectively. The filaments are known to interact with one another, producing tension and/or shortening in both types of muscle. The energy necessary for this effect is obtained by splitting the ATP, catalyzed by actomyosin ATPase. When the skeletal muscle is in the relaxed state the ATPase activity is inhibited because the regulatory proteins, troponin and tropomyosin, suppress the interaction between myosin and actin (1). This inhibitory action is diminished when the intracellular Ca2+ concentration is increased above the very low resting level (< 10-7 M). In skeletal muscle these Ca2+ ions bind to troponin, causing a conformational change of the troponin-tropomyosin system making it possible for myosin to interact with actin sites and contraction starts. Activation of the contractile proteins in smooth muscle is also controlled via [Ca2+]i. It is not yet certain however if this is through a troponin-tropomyosin system similar to that in skeletal muscle or, as has recently been suggested, via a myosin-linked regulation (2).