Final answer:
Sodium ions (Na+) are not typically considered second messengers in G protein-coupled receptor pathways, unlike cAMP, cGMP, IP3, and DAG.
Step-by-step explanation:
The question asks which of the following is NOT a second messenger utilized by G proteins : 1) Ca₂⁺ 2) Na⁺ 3) cyclic GMP 4) phosphatidyl inositol. The main second messengers known to be used in G protein-coupled receptor pathways include cyclic AMP (cAMP), calcium ions (Ca₂⁺), cyclic GMP (cGMP), inositol triphosphate (IP3), and diacylglycerol (DAG). Na⁺ (sodium ions), however, are not typically considered second messengers in this context. Sodium ions play many roles in cellular processes, particularly in establishing the electrochemical gradients that are critical for functions such as nerve impulse transmission and muscle contraction, but they do not serve as second messengers in signal transduction pathways initiated by G protein-coupled receptors.