Final answer:
Smooth muscle cells have two primary types of receptors associated with the sarcoplasmic reticulum: the α- or β-adrenergic receptors, which are involved in regulating intracellular calcium levels and thus muscle contraction.
Step-by-step explanation:
The two possible receptors on the sarcoplasmic reticulum (SR) of smooth muscle are the nicotinic receptor and the α- or β-adrenergic receptors. During smooth muscle contraction, calcium ions (Ca++) are crucial for initiating the contraction process. These calcium ions may enter the smooth muscle cell either through the calcium channels in the sarcolemma during an action potential or may be released from the SR itself.
When the adrenergic receptors are bound by ligands such as adrenaline or noradrenaline, α1 receptors interact with Gq proteins leading to increased levels of intracellular Ca++ which induce smooth muscle contraction. In contrast, β receptors can lead to smooth muscle relaxation through their interaction with Gs proteins and resultant increases in intracellular cAMP.
The nicotinic receptor is typically associated with the neuromuscular junction rather than direct involvement with the SR in smooth muscle. However, since this question pertains to the receptors on the SR, it's important to note that the primary receptors involved in smooth muscle contraction are the adrenergic receptors, which facilitate the release or intake of Ca++ to regulate muscle activity.