A Single Luminally Continuous Sarcoplasmic Reticulum with Apparently Separate Ca2+ Stores in Smooth Muscle

John G Mccarron,Marnie L Olson

doi:10.1074/jbc.m708923200

Abstract

Whether or not the sarcoplasmic reticulum (SR) is a continuous, interconnected network surrounding a single lumen or comprises multiple, separate Ca2+ pools was investigated in voltage-clamped single smooth muscle cells using local photolysis of caged compounds and Ca2+ imaging. The entire SR could be depleted or refilled from one small site via either inositol 1,4,5-trisphosphate receptors (IP3R) or ryanodine receptors (RyR) suggesting the SR is luminally continuous and that Ca2+ may diffuse freely throughout. Notwithstanding, regulation of the opening of RyR and IP3R, by the [Ca2+] within the SR, may create several apparent SR elements with various receptor arrangements. IP3R and RyR may appear to exist entirely on a single store, and there may seem to be additional SR elements that express either only RyR or only IP3R. The various SR receptor arrangements and apparently separate Ca2+ storage elements exist in a single luminally continuous SR entity.

Highlights

The Ca2ϩ storage element of the endoplasmic reticulum (ER), the sarcoplasmic reticulum (SR), may be a specialized region of the SER that is endowed with those proteins responsible for the uptake (Ca2ϩ pumps), storage, and release ryanodine receptor ((RyR) and inositol 1,4,5-trisphosphate receptor (IP3R)) of Ca2ϩ [4, 6]
The Ca2ϩ storage element of the ER, the sarcoplasmic reticulum (SR), may be a specialized region of the SER that is endowed with those proteins responsible for the uptake (Ca2ϩ pumps), storage, and release ryanodine receptor
The SR Ca2ϩ stores have been classified on the basis of the arrangement of IP3R and ryanodine receptors (RyR) and conflicting evidence exists regarding their number and receptor arrangement

Summary

EXPERIMENTAL PROCEDURES

Male guinea pigs (500 –700 g) were humanely killed by cervical dislocation followed by immediate exsanguination in accordance with the guidelines of the Animal (Scientific Procedures) Act UK 1986. Whole cell currents were measured using an Axopatch 200B (Axon Instruments, Union City, CA), low-pass filtered at 500 Hz (8-pole bessel filter; Frequency Devices, Haverhill, MA), digitally sampled at 1.5 kHz using a Digidata interface and pClamp (version 8; Axon Instruments) and stored for analysis. Electrophysiological measurements and imaging data were synchronized by recording, on pClamp, a transistor logic output from the CCD camera, which reported its readout status, together with the electrophysiological information. In those control experiments that involved the use BODIPY fluorescent ryanodine, [Ca2ϩ]c was measured using fura-2 in a microfluorimeter system [34].

A Photolysis

RESULTS

Findings

DISCUSSION