I am trying to improve on my ability to test/calibrate voltage reporting dyes in vivo, in embryonic and larval cells of Xenopus; I have found valinomycin and valinomycin/nigericin cocktails, coupled with changes in K concentration in the medium, to be useful, but less than ideal. I am not interested in electrophysiological techniques, just drugs or other chemicals that I can add to the normal medium (we use 0.1X MMR).
Check out: Schwab A et al (1995). Xenopus Laevis oocyte: Using living cells to teach the theory of cell membrane potential. Advances in Physiology Education 13 (1): S26-31
A nice tutorial
You could try TEA and ouabain ( or in combination) to block Na/K-ATPase (apparently, this current contributes significantly to Em).
See Reference: http://www.sciencedirect.com/science/article/pii/S0005273698000662
Voltage-dependent inhibition of the Na+,K+ pump by tetraethylammonium by Ursula Eckstein-Ludwig et al. (1998). Biochimica et Biophysica Acta (BBA) - Biomembranes Volume 1372, Issue 2, 17 July 1998, Pages 289–300.
Any physiological solution (e.g. frog Ringer) will do. For depolarizing use high potssium in place of sodium. Usually 50 mM KCl and proportionally lower NaCl should completely depolarize the cell. To hyperpolarize, just completely remove potassium from your solution
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