Endogenous Currents

There are a number of ionic currents linked to extracellular receptors that can affect the membrane potential. Stimulation of muscarinic receptors by acetylcholine turns on a Ca ²⁺ activated Cl^- current out of the cell. This is due to the production of the second messenger inositol trisphosphate (IP3) by the muscarinic receptor which releases Ca²⁺ from the endoplasmic reticulum. This intracellular Ca ²⁺ causes the opening of the Cl^- channels.

Activation of this current results in depolarization of the membrane caused by the efflux of Cl^-.

Beta-adrenergic receptor activation can also affect the membrane potential. Beta agonists, such as epinephrine, cause the production of cyclic adenosine monophosphate (cAMP). This causes an activation of an outward K⁺ current.

This results in a further polarization of the membrane, or hyperpolarization .

The oocytes, therefore, possess receptors linked to both the IP₃/Ca²⁺ and cAMP second messenger systems that may be affected by the herbal medicines. The effect of these second messengers can be easily determined by measuring the membrane potential of the oocytes before and after application of the herb extract. Experiments using the herbs with aortic smooth muscle show that many of them have potent effects on these second messenger systems.

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