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Abstract Expression of rat α1G, human α1H and rat α1I subunits of voltage-activated Ca2 + channels in HEK-293 cells yields robust Ca2 + inward currents with 1.25 mm Ca2 + as the charge carrier. Both similarities and marked differences are found between their biophysical properties. Currents induced by expression of α1G show the fastest activation and inactivation kinetics. The α1H and α1I currents activate and inactivate up to 1.5- and 5-fold slower, respectively. No differences in the voltage dependence of steady state inactivation are detected. Currents induced by expression of α1G and α1H deactivate with time constants of up to 6 ms at a test potential of - 80 mV, but currents induced by α1I deactivate about three-fold faster. Recovery from short-term inactivation is more than three-fold slower for currents induced by α1H and α1I in comparison to α1G. In contrast to these characteristics, reactivation after long-term inactivation was fastest for currents arising from expression of α1I and slowest in cells expressing α1H calcium channels. The calcium inward current induced by expression of α1I is increased by positive prepulses while currents induced by α1H and α1G show little ( < 5%) or no facilitation. The data thus provide a characteristic fingerprint of each channel’s activity, which may allow correlation of the α1G, α1H and α1I induced currents with their in vivo counterparts. [ABSTRACT FROM AUTHOR]

The expression of Ca2+ channel α1E isoforms has been analyzed in different cell lines, embryoid bodies and tissues. The comparison of the different cloned α1E cDNA sequences led to the prediction of α1E splice variants. Transcripts of two cloned α1E isoforms, which are discriminated by a carboxy terminal 129-bp sequence, have been detected in different cell lines and tissues. Transcripts of the shorter α1E isoform have been assigned to the rat cerebrum and to neuron-like cells from in vitro, differentiated embryonic stem cells. The shorter isoform is the major transcript amplified from total RNA by reverse transcription (RT)-PCR and visualized on the protein level by Western blotting with common and isoform-specific antibodies. Transcripts of the longer α1E isoform have been identified in mouse, rat and human cerebellum, in in vitro, differentiated embryoid bodies, in the insulinoma cell lines INS-1 (rat) and βTC-3 (mouse), in the pituitary cell line AtT-20 (mouse) when grown in 5 mM glucose, and in islets of Langerhans (rat) and kidney (rat and human). The detection of different isoforms of α1E in cell lines and tissues shows that the wide expression of α1E has to be specified by identifying the corresponding isoforms in each tissue. In islets of Langerhans and in kidney, a distinct isoform called α1Ee has been determined by RT-PCR, while in cerebellum a set of different α1E structures has been detected, which might reflect the functional heterogeneity of cerebellar neurons. The tissue-specific expression of different isoforms might be related to specific functions, which are not yet known, but the expression of the new isoform α1Ee in islets of Langerhans and kidney leads to the suggestion that α1E might be involved in the modulation of the Ca2+-mediated hormone secretion. [ABSTRACT FROM AUTHOR]