Salbutamol (structure below, pKa 9.3):

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Salbutamol is a β2-adrenoceptor agonist with a basic amine and a pKa around 9.3. At physiological pH (~7.4), a significant portion of the molecule will exist in the protonated (ionized) form, reducing its lipophilicity and thus its ability to diffuse across lipid membranes.
Option 1: 'penetrates membranes less readily when in the ionized form' — This is consistent with general medicinal chemistry principles: ionized species have poorer membrane permeability due to decreased lipophilicity, so salbutamol would cross membranes less readily when protonated.
Option 2: 'is predominantly in the neutral form at physiological pH' — Given a pKa of 9.3 for the basic amine, the drug will be largely ionized at pH 7.4, not predominantly neutral. This makes the statement inaccurate.
Option 3: 'has a selective action on heart muscle' — Salbutamol is known for β2-selectivity (bronchodilation) rather than selectivity for heart muscle (which would involve β1 receptors primarily). This option misstates its receptor selectivity.
Option 4: 'is a selective β2-adrenoceptor antagonist' — Salbutamol is an agonist, not an antagonist, and it is selective for β2 receptors. Labeling it as an antagonist is incorrect in mechanism and action.
In summary, the key concept is that the ionized form at physiological pH reduces membrane permeability, which aligns with option 1, while the other statements contain inaccuracies about pH-dependent form, receptor selectivity, or pharmacologic action.

Salbutamol (structure below, pKa 9.3):

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