Ans : D (IP3-DAG) & E (Ca2+ channel) Source of Calcium Ions & in Smooth Muscle Contraction Mechanism InvolvedAlmost all the calcium ions that cause contraction enter the muscle cell from the extracellular fluid at the time of action potential or other stimulusSarcoplasmic reticulum (the source of intracellular Ca2+), which provides virtually all the calcium ions for skeletal muscle contraction, is only slightly developed in most smooth muscle.When an action potential is transmitted in caveolae (rudimentary' analogue of the transverse tubule system of skeletal muscle), this is believe to excite calcium ion release from the abutting sarcoplasmic reticulum tubules through opening of calcium channels (Guyton 11th/99, 90)Sometimes smooth muscle contraction or inhibition is initiated by hormone without directly causing any change in the membrane potential. In these circumstances, the hormone may activate a membrane receptor that does not open any ion channel but instead cause an internal change in the muscle fiber, such as release of Ca2+ from intracellular sarcoplasmic reticulum, the Ca2+ then induce contraction (Guyton 11th/98)IP3-DAG role (to favour as an ansiver)The acetycholine cause smooth muscle contraction by phospholipase C& IP.,, which increase the intracellular Ca2+ concentration-Gcrnong 22nd/83Role of CAMP & CGMP (Guyton 11th/98)CAMP & CGMP so called second messengers has many effects on smooth muscle, one of which is to change the degree of phosphorylation of several enzyme that indirectly inhibit contraction."An increase in CAMP dampens the contraction response of smooth muscle to a given elevation of sarcoplasmic Ca2+-Harper 2 7th/579