Somatotropes and GC cells, a GH-producing cell line, exhibit [Ca2+]i oscillations that result from rhythmic Ca2+ action potentials. Determination of this operating mode required simultaneous recording of both parameters by fura-2 imaging and patch-clamp techniques. In order to test whether patch recording induces artificial alteration of the [Ca2+]i oscillatory pattern, we recorded separately or simultaneously [Ca2+]i and membrane potential. In the absence of any other stimulation, seal formation in patch-clamp recording evoked by itself a 2.5- to 4-fold persistent increase in basal [Ca2+]i, speeded up their frequency (from 0.03–0.17 to 0.4 Hz) and changed their pattern to a tonic mode. Patch-induced [Ca2+]i increase was reproduced by mechanical contact between the pipette and the membrane. It was reduced by nifedipine, a blocker of L-type Ca2+ channels, as well as by removal of external Na+. It was fully blocked by external Ca2+ removal or gadolinium. All patch-clamp-induced perturbations were reversed by membrane hyperpolarization. We propose that patch-clamp recording evokes Ca2+ entry through L-type Ca2+ channels either directly, or indirectly via membrane depolarization. This shows that patch recordings in endocrine cells showing mechanosensitivity have to be interpreted with caution, and explains why long-lasting patch recordings are so difficult to obtain.

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