DesignThe membrane current mediated by the L-type calcium channel (ICa) was studied for myocytes isolated from the ventricle of 10− to 11-week-old spontaneously hypertensive rats (SHR).ResultsCompared with age-matched normotensive Wistar-Kyoto (WKY) or Sprague-Dawley rats, the amplitude of ICawas greater for the SHR. Two observations suggest that the greater ICaof the SHR was not due to hypertrophy. First, the similarity of membrane capacitance for these three strains of rat indicated lack of hypertrophy. Secondly, the amplitude of ICawas also greater for myocytes isolated from the right ventricle of the SHR. The ICaof the SHR was more sensitive to drugs known to activate calcium channels via phosphorylation. Specifically, extracellular application of 1 μmol/l isoprenaline as well as intracellular dialysis with either 1 mmol/l cyclic AMP or with 1 mmol/l adenosine 5′-O-3-thiotriphosphate increased the mean ICaof SHR myocytes significantly more than that of WKY rat cells. The ICaof SHR myocytes was also more sensitive to BAY K 8644 and its enantiomorphs.ConclusionThe present data suggest that the greater peak amplitude of ICafor SHR myocytes relative to that of myocytes of normotensive rats is due to an increase in current density and enhancement of channel phosphorylation.