Superficial Sarcoplasmic Reticulum Regulates Activated and Steady-State Cytosolic Ca²⁺ Concentrations in Vascular Smooth Muscle

Abstract

Force development of depolarized smooth muscle has been found to be more strongly correlated with the rate of Ca²⁺ entry than with net Ca²⁺ uptake [27]. These observations led to the superficial buffer barrier hypothesis, which states that a fraction of the Ca²⁺ entering vascular smooth muscle cells during depolarization is accumulated by sarcoplasmic reticulum located near the cell surface (superficial SR) before reaching the myofilaments. Subsequently, Loutzenhiser and van Breemen [11] were able to dissociate (at least transiently) stimulated Ca²⁺ influx from contraction by a prior depletion of SR Ca²⁺ using norepinephrine (NE) in a Ca²⁺-free solution. Tension did not develop until SR Ca²⁺ levels were restored to near resting concentrations, which suggests that the agonist-releasable Ca²⁺ pool can be refilled by voltage-stimulated Ca²⁺ entry, and supports the superficial buffer barrier hypothesis. In addition, the response of this tissue to U-44069 (a prostaglandin H₂ analogue which causes the release of a fraction of the NE-releasable intracellular Ca²⁺ pool) was tested in the presence of La³⁺ and D600. Both these agents reduced the contractile response to U-44069 by approximately 50%. However, repetitive contractions could be elicited in the presence of D600, while only a single response was obtainable in the presence of La³⁺ which suggested that the agonist-releasable Ca²⁺ pool can be refilled by passive Ca²⁺ entry. A later study by Hwang and van Breemen [5] demonstrated that caffeine releases the Ca²⁺ taken up by the superficial SR during BAY K8644-stimulated Ca²⁺ influx. From these findings it may be concluded that tension development from activation of voltage-gated calcium channels is Ca²⁺ entering the cell, and that this stored Ca²⁺ is at least a part of the agonist-releasable Ca²⁺ pool.