Chapter 16, Problem 17Q

Summary Introduction

To discuss: The consequences of the feature of ryanodine calcium channels present in the sarcoplasmic reticulum in cardiac-muscle cells that unlike IP₃ opened channels, these are opened by the binding of calcium itself.

Introduction: Sarcoplasmic reticulum is a special type of endoplasmic reticulum found in the muscle cells. They have more number of calcium channels than normal. Phospholipase C cleaves the phosphatidyl inositol to generate two secondary messengers: inositol triphosphate (IP₃) and diacylglycerol (DAG). IP₃ then opens the Ca²⁺ ion channels present on the membrane of the endoplasmic reticulum (ER). The above is the case of normal ER. In the case of muscle cells where troponin is present, there is a requirement of higher amount of Ca²⁺ ions, as troponin has high affinity for it and large number of calcium ions are required for muscle contraction and signaling. So, a special type of calcium channel called ryanodine channel is present in the sarcoplasmic reticulum. Ryanodine channel is opened by the presence of calcium ions itself and do not require any secondary messengers.